KMUCAT Paper 2 2025

Explanation: C
1. Set symbols: Let B = allele for black coat, b = allele for light brown coat.
We are told all 20 offspring are black.
2. Dominance from the result:
If black appears in every lamb, black must be dominant over light brown. (If black were recessive, mating a black male with light-brown females would produce many light-brown lambs, not all black.)
3. Genotypes of the parents:
• Light brown females must be bb (recessive phenotype).
• For every lamb to be black, the black male must contribute a B to each offspring → the male must be BB (homozygous dominant).
4. Punnett square (BB × bb):

All offspring are Bb (black phenotype).
5. Why not heterozygous male (Bb)?
Bb (male) × bb (female) would give 50% Bb (black) and 50% bb (light brown)—not all black. (Getting 20/20 black from this cross would be extremely unlikely.)
Conclusion: The observation (“all offspring black”) is explained only if black is dominant and the male is homozygous (BB) → Option C.

Explanation: B
1. Role of the gall bladder:
• The gall bladder does not produce bile; it only stores and releases bile made by the liver.
• Bile contains bile salts which emulsify fats (break large fat globules into smaller droplets).
• This emulsification increases the surface area for lipase enzymes to act, improving fat digestion.
2. If the gall bladder is removed (cholecystectomy):
• The liver still produces bile normally.
• But bile will no longer be stored and concentrated in the gall bladder.
• Instead, bile will drip continuously into the small intestine in small amounts.
• As a result, digestion of large fatty meals becomes less efficient (reduced fat digestion).

Explanation: C
1. Definition of natural selection:
• Natural selection is the process by which individuals with advantageous traits (that improve survival or reproduction) are more likely to survive and pass on those traits to the next generation.
• It is not directed by humans; it happens due to environmental pressures.
2. Look at the options:
• A. Dogs that are friendly to humans → This is due to artificial selection (selective breeding) by humans, not natural selection.
• B. Grapes that contain no seeds → Also a result of artificial selection or genetic modification, not natural selection.
• C. Mosquitoes resistant to insecticides → This is natural selection:
• Some mosquitoes randomly have mutations that make them resistant.
• When insecticides are sprayed, most mosquitoes die, but resistant ones survive.
• Survivors reproduce, passing resistance genes to the next generation.
• Over time, the population becomes resistant.
• D. Onion crops with pleasant taste → Again, this is due to human-controlled breeding, not natural selection.
Correct Answer: C. Mosquitoes that are resistant to insecticides.

Explanation: D
1. Option A:
• “A dominant characteristic is seen in phenotype of a heterozygote” → Correct.
• Example: In pea plants, Tt (tall) shows the dominant trait “tall”.
2. Option B:
• “A homozygous genotype may be either dominant or recessive” → Correct.
• Example: TT (homozygous dominant) or tt (homozygous recessive).
3. Option C:
• “Recessive phenotypes always have two recessive alleles” → Correct.
• Example: Short pea plants = tt only.
4. Option D:
• “The phenotype of a homozygote is always dominant” → Incorrect.
• Because a homozygote can be dominant (TT → tall) or recessive (tt → short).
• So the phenotype of a homozygote is not always dominant.
Correct Answer: D. The phenotype of a homozygote is always dominant.

Explanation: A
1. Respiration’s role:
• Cellular respiration provides ATP, the energy source for processes that need energy.
• Processes that require ATP → depend on respiration.
• Processes that do not need ATP (passive) → do not depend on respiration.
2. Option A: Diffusion of glucose
• Diffusion is a passive process (down a concentration gradient).
• No ATP is required.
• Does not depend on respiration.
3. Option B: Active uptake of ions
• “Active” = energy required (ATP from respiration).
• Depends on respiration.
4. Option C: Conduction of nerve impulses
• Nerve impulses rely on active transport (Na⁺/K⁺ pump) to restore ionic gradients.
• Requires ATP → depends on respiration.
5. Option D: Muscle contraction
• Contraction requires ATP for actin-myosin interaction and calcium ion pumps.
• Depends on respiration.
Correct Answer: A. Diffusion of glucose.

Explanation: B
1. How mRNA vaccines work:
• They contain a synthetic strand of messenger RNA encoding a viral protein (like the spike protein of SARS-CoV-2).
• Once injected, host cells use this mRNA to make the protein, which triggers an immune response.
2. Option A:
• “They integrate into the host genome for longer immunity” → Incorrect.
• mRNA does not enter the nucleus and does not integrate into DNA.
3. Option B:
• “They can be rapidly designed and produced at scale” → Correct.
• Once the genetic sequence of a pathogen is known, mRNA vaccines can be designed in weeks and manufactured quickly.
• This is a major advantage over traditional vaccines, which may require growing large amounts of virus or protein.
4. Option C:
• “They always eliminate the need for booster doses” → Incorrect.
• Some mRNA vaccines (like COVID-19 vaccines) do require boosters for lasting immunity.
5. Option D:
• “They require live pathogens to work with” → Incorrect.
• mRNA vaccines are pathogen-free; no live virus is used.
Correct Answer: B. They can be rapidly designed and produced at scale.

Explanation: D
Syphilis develops in stages if untreated:
1. Primary syphilis:
• Symptom: A hard, painless red ulcer (chancre) at the site of infection.
→ Matches Option B, but this occurs early (within weeks).
2. Secondary syphilis:
• Symptom: A red skin rash, often on palms and soles, sometimes with fever or swollen lymph nodes.
→ Matches Option C, but this happens months after infection, not years.
3. Latent syphilis:
• No visible symptoms; bacteria remain hidden in the body.
4. Tertiary syphilis (many years later):
• Serious complications if still untreated.
• Symptom: Damage to the central nervous system (neurosyphilis), heart, blood vessels, bones, etc.
→ Matches Option D.
5. Option A (discharge):
• This is more typical of gonorrhea or chlamydia, not late syphilis.
Correct Answer: D. Damage to the central nervous system.

Explanation: A
1. Role of the nervous system:
• The nervous system directly controls rapid responses such as muscle movement, reflexes, gland secretion, and functions of smooth/cardiac muscle.
2. Option A: Blood glucose concentration
• This is mainly regulated by hormones (insulin and glucagon from the pancreas).
• Not under direct nervous system control.
3. Option B: Dilation of blood vessels
• Controlled by the autonomic nervous system (sympathetic/parasympathetic) acting on smooth muscle in vessel walls.
4. Option C: Heart rate
• Directly controlled by the autonomic nervous system via the sinoatrial (SA) node.
5. Option D: Pupil diameter in the eye
• Controlled by the autonomic nervous system (sympathetic → dilation, parasympathetic → constriction).
Correct Answer: A. Blood glucose concentration.

Explanation: C
1. Stage 1: Acute HIV infection
• Happens 2–4 weeks after exposure.
• Flu-like symptoms (fever, rash, sore throat, swollen nodes).
• High viral load, very infectious.
2. Stage 2: Clinical latency (chronic HIV infection)
• Virus is still active but reproduces at low levels.
• Person may have no symptoms or only mild ones.
• Can last years if untreated.
3. Stage 3: AIDS (Acquired Immunodeficiency Syndrome)
• Final stage, where the immune system is badly damaged.
• CD4 cell count drops below 200 cells/µL.
• Person develops opportunistic infections and cancers.
Why not the other options?
A. One stage only: AIDS → Wrong, AIDS is only the last stage.
B. Two stages: acute infection and AIDS → Wrong, it ignores the long latency phase.
D. Four stages: … and full recovery → Wrong, there is no full recovery; HIV is lifelong (though controlled with ART).
Correct Answer: C. Three stages: acute infection, clinical latency and AIDS.

Explanation: C
1. Nature of viruses:
• Viruses have either DNA or RNA as their genetic material, but never both.
• They may be single-stranded or double-stranded, depending on the virus type.
2. Check each option:
A. Smallpox is an RNA virus → Incorrect.
• Smallpox (variola virus, family Poxviridae) is a large, double-stranded DNA virus.
B. Bacteriophages always contain RNA →  Incorrect.
• Most bacteriophages (like T4 phage) have DNA genomes, not RNA.
C. Smallpox is a DNA virus →  Correct.
D. Plant viruses never contain DNA →  Incorrect.
• Most plant viruses are RNA viruses (e.g., Tobacco mosaic virus = RNA), but some plant viruses do contain DNA (e.g., Geminiviruses).
Correct Answer: C. Smallpox is a DNA virus.

Explanation: D
1. Proteins in diet:
• The body cannot make all the essential amino acids.
• Proteins in food are broken down into amino acids → used to build new proteins for growth, repair, and enzymes.
2. Check each option:
A. They are an energy source → Not their main role. Although proteins can be used for energy (4 kcal/g), this happens mainly in starvation, not their essential function.
B. They are used in synthesizing hormones → Partly true (many hormones like insulin are proteins), but this is not the primary reason they are essential.
C. They can be stored as a food reserve → Proteins are not stored; excess amino acids are converted to fat or glucose.
D. They provide amino acids for cell growth → Correct. This is the fundamental reason proteins are essential.
Correct Answer: D. They provide amino acids for cell growth

Explanation: B
1. Bone strength depends on:
• Calcium → the main mineral that forms the hard matrix of bones and teeth.
• Vitamin D → helps in absorption of calcium from the intestine and its deposition in bones.
2. Check the options:
A. Calcium & Vit C → Vitamin C is important for collagen formation (connective tissue), but not the main partner for calcium in bone hardening.
B. Calcium & Vit D → Correct. Classic example: deficiency causes rickets in children and osteomalacia in adults.
C. Zinc & Vit C → Important for wound healing and immunity, not directly for strong bones.

Explanation: D
1. Effect of tobacco smoke:
• It contains harmful chemicals that trigger inflammation and release of enzymes which break down elastic fibers in alveolar walls.
• Alveoli lose elasticity → lungs cannot recoil properly → air gets trapped.
2. Consequences:
• The alveolar walls may rupture, reducing surface area for gas exchange.
• Breathing becomes difficult, especially exhalation.
• Characteristic symptom: “barrel chest” and chronic breathlessness.
3. Why not the other options?
A. Bronchitis → inflammation and mucus buildup in bronchi, but alveolar wall damage is not the main issue.
B. Lung cancer → uncontrolled growth of abnormal cells; not primarily elastic tissue breakdown.
C. Pneumonia → infection causing alveoli to fill with fluid/pus, not loss of elasticity.
D. Emphysema → Correct; specifically caused by destruction of elastic alveolar tissue.
Correct Answer: D. Emphysema.

Explanation: A
1. Hypothalamus:
• Acts as the thermoregulatory center of the brain.
• Detects changes in blood temperature and receives signals from skin thermoreceptors.
• Triggers responses like sweating, shivering, vasodilation, and vasoconstriction to maintain homeostasis.
2. Other options:
B. Cerebellum → Controls balance, coordination, and posture, not temperature.
C. Medulla (medulla oblongata) → Controls vital involuntary functions like heartbeat, breathing, and blood pressure.
D. Cerebrum → Involved in higher thinking, memory, emotions, and voluntary actions, not direct temperature control.
Correct Answer: A. Hypothalamus.

Explanation: B
1. Function of hypothalamus:
• It is the homeostatic control center of the brain.
• Monitors changes in the concentration of solutes in blood (osmolarity), blood temperature, and levels of CO₂.
• Contains osmoreceptors that detect changes in water balance.
• If blood becomes too concentrated, it signals the pituitary gland to release ADH (antidiuretic hormone).
2. Why not the others?
A. Cerebellum → Controls coordination, balance, and posture, not blood concentration.
C. Medulla → Controls heartbeat, breathing, and reflexes like swallowing; it does monitor CO₂ for breathing rate, but solute concentration/osmotic balance is primarily regulated by the hypothalamus.
D. Pituitary gland → Releases hormones (like ADH), but it acts on signals from the hypothalamus.
Correct Answer: B. Hypothalamus.

Explanation: C
1. When body temperature drops:
• The hypothalamus activates mechanisms to generate and conserve heat.
2. Responses:
• Shivering: Rapid, involuntary muscle contractions that produce heat through respiration.
• Vasoconstriction (of skin blood vessels): Narrowing of blood vessels near the skin surface reduces blood flow → less heat lost to the environment.
3. Why not the other options?
A. Shivering + vasodilation → Vasodilation increases heat loss, which would worsen cooling.
B. Sweating + vasoconstriction → Sweating reduces body temperature, not correct in cold conditions.
D. Sweating + vasodilation → Both are mechanisms for cooling, not warming.
Correct Answer: C. Shivering and vasoconstriction in blood vessels near the skin surface.

Explanation: A
1. Aorta:
• Carries blood from the left ventricle to the body.
• Blood is oxygen-rich (just pumped from lungs via left atrium/ventricle).
• CO₂ is low.
• Pressure is highest because it receives blood directly from the left ventricle’s powerful contraction.
2. Pulmonary artery:
• Carries blood from right ventricle → lungs.
• Blood is deoxygenated (low O₂, high CO₂).
3. Pulmonary vein:
• Carries blood from lungs → left atrium.
• Blood is oxygenated and low in CO₂, but pressure is much lower than in the aorta.
4. Vena cava:
• Carries blood from body → right atrium.
• Blood is low in O₂, high in CO₂, and under low pressure. 
Correct Answer: A. Aorta.

Explanation: B
1. Right ventricle contraction (systole):
• Blood is pumped from the right ventricle → pulmonary artery through the pulmonary valve.
• To prevent backflow into the right atrium, the tricuspid valve (between right atrium and right ventricle) must be closed.
2. Check the options:
A. Bicuspid valve (mitral valve): Found between the left atrium and left ventricle, not involved in right ventricle contraction.
B. Tricuspid valve: Located between right atrium and right ventricle → must close during right ventricular systole.
C. Aortic valve: Found at the exit of the left ventricle → not related here.
D. Pulmonary valve: This one opens during right ventricular contraction to allow blood into pulmonary artery, not close. 
Correct Answer: B. Tricuspid valve

Explanation: B
1. What happens after a protein-rich meal?
• Proteins → digested into amino acids → absorbed into blood via small intestine.
• Amino acids enter the hepatic portal vein → transported to the liver.
2. In the liver:
• Excess amino acids undergo deamination (removal of amino group).
• Amino group (–NH₂) is converted into urea (in the urea cycle).
• Urea is then released into the blood leaving the liver.
3. Check each option:
A. Hepatic portal vein → High in amino acids after the meal, but not yet high in urea.
B. Hepatic vein → Carries blood away from the liver after deamination → highest in urea.
C. Renal artery → Brings urea-rich blood from body to kidney, but urea concentration is slightly lower than in hepatic vein (since urea is diluted by systemic circulation).
D. Renal vein → Carries blood away from kidney after filtration, so urea concentration is lowest.
Correct Answer: B. Hepatic vein.

Explanation: D
1. What is negative feedback?
• A control mechanism where a change in a condition triggers a response that reverses (negates) that change, keeping conditions stable (homeostasis).
2. In this case (osmoregulation):
• If water in blood falls → hypothalamus detects high solute concentration → pituitary releases more ADH.
• ADH makes kidneys reabsorb more water → urine becomes more concentrated (less water lost).
• This restores blood water concentration back to normal.
3. Why is it negative feedback?
• Because the body reverses the initial change (low blood water → response increases blood water again).
4. Check options:
A. It decreases the amount of water in blood → Wrong, it restores it.
B. It increases any change → That describes positive feedback, not negative.
C. It increases the amount of water in the blood → Incomplete; it doesn’t always “increase,” it restores balance.
D. It reverses any change in the amount of water in the blood → Correct.
Correct Answer: D. It reverses any change in the amount of water in the blood.

Explanation: B
1. Principle of dialysis:
• A dialysis membrane is partially permeable.
• Small molecules (like urea, salts, glucose) can diffuse across it.
• Large molecules (like proteins, hormones, blood cells) cannot cross.
2. Check each option:
A. Glucose → Small molecule, can diffuse if concentration gradient exists.
B. Insulin → It is a protein hormone (though relatively small compared to large proteins, still too big to pass freely through the dialysis membrane).
C. Sodium → Small ion, diffuses easily.
D. Urea → Small waste molecule, main target for removal in dialysis.
Correct Answer: B. Insulin

Explanation: D
1. Sequence of events:
• The loud noise is detected by receptors in the ear.
• A sensory neuron carries this information as an impulse to the central nervous system (CNS).
• Inside the CNS, the impulse passes to a relay (interneuron), which processes and connects sensory input to the appropriate response pathway.
• The relay neuron transmits the signal to a motor neuron, which carries impulses to muscles (e.g., vocal cords, diaphragm) to make the child shout for help.
2. Check options
A. Motor → relay → sensory wrong order (backwards).
B. Motor → sensory → relay wrong, action doesn’t come before detection.
C. Sensory → motor → relay wrong, relay comes before motor in the CNS.
D. Sensory → relay → motor correct order.
Correct Answer: D. Sensory neuron → relay neuron → motor neuron.

Explanation: A
1. Role of the hypothalamus:
• It contains osmoreceptors that detect changes in blood water concentration (osmotic pressure).
• If blood is too concentrated → hypothalamus signals pituitary gland to release ADH.
• ADH makes kidneys reabsorb more water → restores balance.
• If blood is too dilute → less ADH released → more water lost in urine.
• Therefore, hypothalamus is the control center for water balance.
2. Other options:
B. Cerebellum → Controls coordination and balance, not fluid regulation.
C. Cerebrum → Higher thinking, memory, voluntary actions.
D. Medulla → Controls heartbeat, breathing, blood pressure, but not water balance.
Correct Answer: A. Hypothalamus.

Explanation: A
1. Reflex arc pathway:
• Receptor detects stimulus (e.g., pain).
• Sensory neuron carries impulse → CNS (spinal cord/brain).
• Relay neuron processes signal inside CNS.
• Motor neuron carries impulse from CNS to an effector (muscle or gland).
• Effector produces response (e.g., muscle contracts).
2. Check the options:
A. Carrying nerve impulses from the central nervous system to an effector → This is the exact function of motor neurons.
B. Connecting a receptor to the central nervous system → That’s the job of sensory neurons.
C. Forming a synapse with a sensory neuron → Relay neurons connect sensory and motor neurons.
D. Transferring energy from the stimulus to a nerve impulse → That’s the role of receptors, not motor neurons.
Correct Answer: A. Carrying nerve impulses from the central nervous system to an effector.

Explanation: C
1. Types of enzyme inhibition:
• Competitive inhibition: Inhibitor competes with substrate for the active site.
• Non-competitive inhibition: Inhibitor binds to an allosteric site (a site other than the active site).
2. Effect of non-competitive inhibition:
• Changes the shape of the enzyme (including active site).
• Substrate can still bind (sometimes), but enzyme cannot catalyze reaction effectively.
• Rate of reaction decreases regardless of substrate concentration.
3. Check options:
A. Competing with the substrate at the active site → This is competitive inhibition.
B. Increasing substrate concentration → This increases rate, not decreases it.
C. Binding to a site other than the active site → Correct, definition of non-competitive inhibition.
D. Denaturing the enzyme → Denaturation is caused by heat/pH extremes, not inhibitors.
Correct Answer: C. Binding to a site other than the active site.

Explanation: C
1. Structure of DNA:
• DNA is a double helix made of repeating units called nucleotides.
• Each nucleotide = deoxyribose sugar + phosphate group + nitrogenous base.
2. Backbone composition:
• The sugar (deoxyribose) and phosphate groups link together in a chain.
• Nitrogenous bases (A, T, G, C) project inward and pair by hydrogen bonds.
• Thus, the backbone = deoxyribose + phosphate.
3. Check options:
A. Nucleotides and amino acids → Amino acids make proteins, not DNA backbone.
B. Ribose and phosphate → Ribose is in RNA, not DNA.
C. Deoxyribose and phosphate → Correct.
D. Bases and deoxyribose → Bases are not part of the backbone; they are the rungs of the ladder.
Correct Answer: C. Deoxyribose and phosphate.

Explanation: A
1. Metabolism of fatty acids:
• Fatty acids are broken down in the mitochondria by a process called β-oxidation.
• This process sequentially removes 2-carbon fragments from the fatty acid chain.
2. Product of β-oxidation:
• Each 2-carbon fragment is converted into Acetyl-CoA.
• Acetyl-CoA then enters the Krebs cycle (citric acid cycle) to produce NADH, FADH₂, and ATP.
3. Check options:
A. Acetyl-CoA → Correct; direct product of β-oxidation.
B. Glucose → Fatty acids cannot be converted back into glucose (except glycerol part of triglycerides).
C. Pyruvate → Pyruvate is formed from glucose (glycolysis), not fatty acids.
D. Citrate → Citrate is formed inside the Krebs cycle from Acetyl-CoA + oxaloacetate, not directly from fatty acids.
Correct Answer: A. Acetyl-CoA.

Explanation: B
1. Darwin’s theory of evolution (1859, On the Origin of Species):
• Populations show variation.
• More offspring are produced than can survive (struggle for existence).
• Individuals with favorable traits survive and reproduce more.
• Over generations, these advantageous traits become common.
• This process = natural selection.
2. Check options:
A. Mutation in genes → Mutations create variation, but Darwin did not know about genes or mutations.
B. Natural selection → Darwin’s main principle.
C. Use and disuse of organs → This was Lamarck’s theory, not Darwin’s.
D. Inheritance of acquired characteristics → Also Lamarck’s idea (e.g., giraffes stretching necks).
Correct Answer: B. Natural selection.

Explanation: D
1. Human skull bones overview:
• The skull has cranial bones (protect the brain) and facial bones.
• Cranial bones include: frontal, parietal, temporal, occipital, sphenoid, ethmoid.
2. Question focus:
• Which bone contributes to both the cranial floor and the lateral walls?
3. Check each option:
A. Occipital bone → Forms the posterior base of skull and part of cranial floor, but not lateral walls.
B. Frontal bone → Forms the forehead and part of the roof of the cranium, not the lateral walls or much of the floor.
C. Sphenoid bone → Important “keystone” bone of the skull, contributes to cranial floor and some side portions near temples, but it is mainly internal, not the major lateral wall bone.
D. Temporal bone → Forms the sides (lateral walls) of skull and also a significant portion of the cranial floor (middle cranial fossa).
Correct Answer: D. Temporal bone.

Explanation: A
1. Vertebral column regions:
• Cervical (7) → Neck, small vertebrae, no rib articulation.
• Thoracic (12) → Chest, vertebrae have facets for rib articulation.
• Lumbar (5) → Lower back, large bodies for weight support, no rib facets.
• Sacral (5 fused) → Form sacrum, articulate with pelvis, not ribs.
2. Why thoracic vertebrae?
• Each thoracic vertebra has costal facets (on body and transverse processes).
• These facets form joints with rib heads and tubercles.
• This allows rib cage to attach and support breathing movements.
3. Check options:
A. Thoracic region → Correct, articulates with ribs.
B. Lumbar region → Large vertebrae for weight bearing, no rib connection.
C. Sacral region → Fused to pelvis, not ribs.
D. Cervical region → Only supports head and neck.
Correct Answer: A. Thoracic region.

Explanation: A
1. Biceps muscle action:
• The biceps brachii is a flexor muscle of the upper arm.
• When it contracts, it pulls the radius bone upwards at the elbow joint.
• This causes the forearm to bend toward the upper arm (flexion at the elbow).
2. Check options:
A. Arm bends → Correct, contraction of biceps = flexion.
B. Arm straightens → Straightening (extension) is caused by the triceps muscle, not biceps.
C. Arm rotates → Rotation involves shoulder joint muscles (like deltoid, rotator cuff).
D. Nothing happens → Wrong, contraction of biceps definitely produces movement.
Correct Answer: A. Arm bends.

Explanation: C
1. Genetic basis of haemophilia:
• Haemophilia is caused by a recessive allele on the X chromosome.
• Symbol:
Xᴴ = normal allele
Xʰ = haemophilia allele
2. Parents’ genotypes:
Female carrier = XᴴXʰ
Normal male = XᴴY
3. Cross (Punnett square):

Result:
• Daughters: 50% normal, 50% carriers (but not diseased).
• Sons: 50% normal, 50% haemophiliac.
5. Answer to question:
• The probability that a son will be haemophiliac = 50% 
Correct Answer: C. 50%.

Explanation: C
1. Nature of haemophilia:
• It is an X-linked recessive disorder.
• The faulty gene is carried on the X chromosome.
2. Males (XY):
• If the single X has the defective allele (XʰY), there is no second X to mask it.
→ Males express haemophilia more often.
3. Females (XX):
• For a female to express haemophilia, both X chromosomes must carry the defective allele (XʰXʰ).
• If only one X carries it (XᴴXʰ), the normal allele on the other X masks the effect → female becomes a carrier but not diseased.
4. Check options:
A. The gene is present on autosomes → Wrong, it is on the X chromosome, not autosomes.
B. They have a stronger immune system → Not relevant to genetic inheritance.
C. They require two defective X-chromosomes to express the trait → Correct explanation.
D. Hormonal protection suppresses the gene → Hormones are not involved here.
Correct Answer: C. They require two defective X-chromosomes to express the trait.

Explanation: D
1. Traditional method (before genetic engineering):
• Insulin was extracted from cow or pig pancreas.
• Problem: slight structural differences → allergic reactions in some patients.
2. Modern method (recombinant DNA technology):
• Scientists insert the human insulin gene into a plasmid (a small circular DNA in bacteria, usually E. coli).
• Bacteria reproduce rapidly and use the human gene to produce human insulin protein.
• This insulin is purified and used for diabetic patients.
3. Check options:
A. Extracting ox-/cow-pancreas → Old method, not used commercially today.
B. Extracting insulin from human pancreas → Impossible and unethical.
C. Inserting a bacterial gene into human pancreas → Backwards; we insert human gene into bacteria, not the other way.
D. Inserting the human insulin gene into a bacterium → Correct, modern commercial method.
Correct Answer: D. Inserting the human insulin gene into a bacterium.

Explanation: B
1. DNA vs RNA viruses:
• DNA viruses: Have DNA as their genetic material (single- or double-stranded).
• RNA viruses: Have RNA as their genetic material (single- or double-stranded).
2. Check the options:
A. Influenza → Caused by Influenza virus, an RNA virus (Orthomyxoviridae).
B. Hepatitis B → Caused by Hepatitis B virus (HBV), a DNA virus (Hepadnaviridae, partially double-stranded DNA).
C. Measles → Caused by Measles virus, an RNA virus (Paramyxoviridae).
D. COVID-19 → Caused by SARS-CoV-2, an RNA virus (Coronaviridae).
3. Key fact: Hepatitis B is one of the very few DNA viruses that cause chronic human infection.
Correct Answer: B. Hepatitis B.

Explanation: C
1. Causative agent of AIDS:
• AIDS (Acquired Immunodeficiency Syndrome) is caused by HIV (Human Immunodeficiency Virus).
• HIV is a retrovirus (RNA virus).
2. Target cells:
• HIV specifically attacks Helper T-lymphocytes (CD₄⁺ T-cells).
• These cells are critical for coordinating the immune response.
3. Effect of infection:
• Destruction of helper T-cells → weakened immune system.
• Body becomes vulnerable to opportunistic infections and certain cancers.
4. Check options:
A. Red blood cells → HIV does not infect RBCs; they lack a nucleus and CD4 receptors.
B. Neurons → Not the main target; though HIV can indirectly affect the nervous system, it does not primarily infect neurons.
C. Helper T-lymphocytes → Correct, main target cells.
D. Platelets → Platelets are cell fragments, not infected by HIV. 

Explanation: D
1. Types of immunity:
• Active immunity: Body’s own immune system produces antibodies and memory cells after exposure to antigen. Long-lasting.
• Passive immunity: Pre-formed antibodies are received from outside. Short-term.
2 Check options:
A. Vaccination → This is artificially acquired active immunity, because antigens are introduced by injection.
B. Breastfeeding → This is naturally acquired passive immunity, because antibodies (IgA) are passed from mother to baby.
C. Injection of antibodies → This is artificially acquired passive immunity (e.g., antivenom, immunoglobulin therapy).
D. Recovering after a viral infection → This is naturally acquired active immunity, because your body makes its own antibodies and memory cells after natural exposure.
Correct Answer: D. Recovering after a viral infection.

Explanation: D
1. Role of B-lymphocytes:
• B-cells are a type of lymphocyte that mature in the bone marrow.
• When activated by an antigen (often with help from helper T-cells), they differentiate into plasma cells.
• Plasma cells produce and secrete antibodies (immunoglobulins) specific to the antigen.
2. Check other options:
A. T-lymphocytes → They regulate immunity (helper T-cells) or kill infected cells (cytotoxic T-cells), but they do not produce antibodies.
B. Neutrophils → These are phagocytes that engulf pathogens, not antibody producers.
C. Macrophages → Also phagocytes; they present antigens but don’t make antibodies.
D. B-lymphocytes → Correct; they are the source of antibody production.
Correct Answer: D. B-lymphocytes.

Explanation: B
1. Placenta formation:
• The placenta develops when embryonic tissues (chorionic villi) attach to the uterine wall of the mother.
• It connects mother’s blood supply with fetal blood (without direct mixing).
2. Function of placenta:
• Allows exchange of oxygen, carbon dioxide, nutrients, urea, hormones, and antibodies between mother and fetus.
• Waste moves from fetus → mother, nutrients and oxygen move from mother → fetus.
3. Check options:
A. Oviduct wall → Fertilization/early development happens here, not placental exchange.
B. Uterus wall → Correct, placenta attaches here for exchange.
C. Cervical wall → Cervix is the lower opening of the uterus, no placental exchange.
D. Umbilical cord → Umbilical cord connects fetus to placenta, but exchange occurs at placenta–uterus interface, not in the cord itself.
Correct Answer: B. Uterus wall.

Explanation: D
1. Before ovulation:
• Rising FSH stimulates follicle growth.
• Follicle produces oestrogen, which thickens the uterine lining.
• A surge in LH (and a smaller FSH rise) triggers ovulation.
2. After ovulation (luteal phase):
• The ruptured follicle becomes the corpus luteum.
• Corpus luteum secretes large amounts of progesterone (and some oestrogen).
• Progesterone maintains the uterine lining for possible implantation.
3. Three days after ovulation:
• Progesterone levels rise significantly compared with pre-ovulation days.
4. Check options:
A. FSH → Falls after ovulation.
B. LH → Peaks at ovulation, then declines.
C. Oestrogen → Moderate rise, but not the major hormone post-ovulation.
D. Progesterone → Major hormone secreted after ovulation.
Correct Answer: D. Progesterone.

Explanation: A
1. Amniotic fluid:
• Clear, watery liquid that surrounds the fetus inside the amniotic sac.
• Provides a protective cushion against shocks, allows free movement, prevents dehydration, and maintains a stable temperature.
2. Other options:
• B. Plasma → Fluid part of blood, not surrounding fetus.
• C. Cerebrospinal fluid → Surrounds the brain and spinal cord, not the fetus.
• D. Pleural fluid → Found between pleural membranes around the lungs, not in pregnancy.
Correct Answer: A. Amniotic fluid

Explanation: C
1. Vena cava (superior & inferior):
• Bring deoxygenated blood from the body → right atrium.
2. Pulmonary artery:
• Carries deoxygenated blood from right ventricle → lungs for oxygenation.
3. Check other options:
A. Aorta + pulmonary artery → Aorta carries oxygenated blood.
B. Aorta + pulmonary vein → Both carry oxygenated blood.
C. Vena cava + pulmonary artery → Both carry deoxygenated blood.
D. Vena cava + pulmonary vein → Pulmonary vein carries oxygenated blood from lungs to left atrium.
Correct Answer: C. Vena cava and pulmonary artery.

Explanation: D
1. Types of variation:
• Continuous variation:
→ Shows a wide range with intermediate values.
→ Controlled by many genes (polygenic) + environment.
→ Examples: height, skin colour, body mass.
• Discontinuous variation:
→ Falls into distinct categories, no intermediates.
• Usually controlled by a single gene.
• Examples: blood groups, tongue rolling, genetic disorders.
2. Check options:
A. Height → Continuous (many intermediate values).
B. Skin colour → Continuous (many shades, polygenic).
C. Agility → Continuous (varies by degree, not categories).
D. Sickle-cell anaemia (present/absent) → Discontinuous (either you have it or you don’t)
Correct Answer: D. Sickle-cell anaemia (present/absent).

Explanation: A
1. Triceps muscle action:
• The triceps is an extensor muscle at the back of the upper arm.
• When it contracts, it straightens (extends) the arm at the elbow
2. Antagonistic muscle pairs:
• Muscles work in pairs because they can only pull, not push.
• When the triceps contracts, the biceps (flexor) must relax.
• When the biceps contracts, the triceps relaxes (to bend the arm).
3. Check options:
A. The biceps muscle begins to relax → Correct, antagonistic action.
B. It starts to pull the radius bone upwards → That’s the action of the biceps, not triceps.
C. The arm becomes bent at the elbow joint → That’s also due to biceps contraction, not triceps.
D. The upper arm is raised above the shoulder → That involves shoulder muscles (like deltoid), not triceps.
Correct Answer: A. The biceps muscle begins to relax.

Explanation: B
1. Equilibrium position:
• The equilibrium position is the relative amounts of reactants and products in a reversible reaction once equilibrium is reached.
• According to Le Chatelier’s Principle, it can shift due to concentration, pressure, or temperature changes.
2. Check options:
A. Change in pressure → Affects reactions involving gases; equilibrium shifts to side with fewer or more moles of gas.
B. Addition of a catalyst → A catalyst only speeds up the rate of forward and backward reactions equally.
• It helps equilibrium to be reached faster but does not change the equilibrium position.
C. Change in concentration → Shifts equilibrium to oppose the change (e.g., adding reactant shifts toward products).
D. Change in temperature → Shifts equilibrium depending on whether the reaction is exothermic or endothermic.
Correct Answer: B. Addition of a catalyst.

Explanation: A
1. Humerus (upper arm bone):
• At its proximal end (near shoulder):
→ The head of the humerus fits into the glenoid cavity of the scapula → forms a ball-and-socket joint (shoulder joint).
• At its distal end (near elbow):
→ The humerus articulates with the ulna and radius → forms a hinge joint (elbow joint).
2. Other options:
B. Radius → Forms part of elbow (hinge with humerus) and wrist joint, but no ball-and-socket.
C. Scapula → Provides socket (glenoid cavity) for humerus, but does not itself form hinge joint.
D. Ulna → Forms hinge joint at elbow, but not a socket joint.
Correct Answer: A. Humerus.

Explanation: B
Breakdown:
• Man: 1
• His five wives: 5 → subtotal 6
• His seven daughters: +7 → 13
• His six sons: +6 → 19
• Sons’ wives: +6 → 25
• Each son has 2 children → 6 × 2 = 12 grandchildren → 25 + 12 = 37.

Explanation: D
• The number increases by 1 each term: 1, 2, 3, 4 → so the middle is 4.
• The first letter steps forward alternately by +3, +4, +3:
D → G (+3), G → K (+4), K → N (+3).
• The last letter advances by decreasing steps +6, +3, +1:
F → L (+6), L → O (+3), O → P (+1).
Hence the next term is N4P.

Explanation: D
Football, tennis, and hockey are typically outdoor field/court sports involving running. Billiards is an indoor cue sport played on a table, not a field or court, and involves no running—so it’s the odd one out.

Explanation: C
We know Azra > Munir and Javaid > Munir, but there’s no info comparing Azra vs Javaid. Either could be older (or same age). So we can’t determine their order.

Explanation: A
Because 30% of 50 = 15 (hockey) and 80% of 50 = 40 (cricket).
By inclusion–exclusion:
∣H ∩ C∣ = ∣H∣ + ∣C∣ − ∣H ∪ C∣ ≥ 15 + 40 – 50 = 5∣H ∩ C∣ = ∣H∣ + ∣C∣ − ∣H ∪ C∣ ≥ 15 + 40 – 50 = 5.
So at least 5 players must play both.

Explanation: C
Onomatopoeia is when a word imitates a sound (e.g., buzz, hiss, clap).
• Simile compares using like/as (e.g., “brave as a lion”).
• Alliteration repeats initial consonant sounds (wild and woolly).
• Metaphor is a direct comparison (time is a thief).

Explanation: C
Etiquette (E-T-I-Q-U-E-T-T-E) is the correct spelling for accepted social manners.
Quick tip: think “eti-QUETTE” with double T and -ette ending (like cassette).

Explanation: B
With neither…nor, the verb agrees with the subject closest to it. The nearer subject is “the teacher” (singular), so it should be “is”, not “are”:
“Neither the students nor the teacher is to blame …”
The others are correct:
• A: singular subjects → is
• C: One … is
• D: data treated as plural → were.

Explanation: A
The wind is given a human action (“whispered”).
• Onomatopoeia: sound-imitating words (e.g., buzz).
• Hyperbole: deliberate exaggeration.
• Simile: comparison using like/as.

Explanation: B
“Quickly” modifies the verb finished, telling how she finished. Words that modify verbs, adjectives, or other adverbs are adverbs.

Explanation: B

Explanation: D
The statement “heat cannot by itself flow from a colder to a hotter body” is the Clausius statement of the second law of thermodynamics.
• The first law concerns energy conservation and does not dictate the direction of heat flow.
• The second law sets the natural direction: spontaneous heat flow is from hot → cold unless external work is done.

Explanation: A
By definition, conventional current is taken in the direction positive charges would move. In metals the actual carriers are electrons (negative) moving the opposite way, but current direction is still defined as if positives were flowing.

Explanation: B
1/R_eq = 1/2 + 1/3 + 1/6 = 0.5 + 0.333…… + 0.166 …… = 1.0 ⇒ R_eq = 1.

Explanation: B
Faraday’s law states that a changing magnetic flux through a loop induces an emf in it (magnitude ∝ rate of change of flux).
• Lenz’s law gives the direction of the induced emf (opposes the change).
• Ampere’s law relates magnetic fields to electric currents.
• Gauss’s law relates electric flux to enclosed charge.

Explanation: A
• LEDs conduct and emit light only when forward biased.
• Photodiodes typically operate reverse biased (photoconductive mode) or at zero bias (photovoltaic mode).
• Photovoltaic cells generate voltage with no external bias.
• LDRs are resistive sensors (not p–n junctions), so “forward bias” doesn’t apply.

Explanation: C
Einstein (1905) explained the photoelectric effect by proposing that light comes in discrete packets (photons) with energy E = hf. This accounts for the observations:
• Threshold frequency: No electrons emitted if f < f₀  because photon energy hf is insufficient to overcome the work function ϕ.
• Instantaneous emission: Absorption of a single photon ejects an electron immediately.
• Kinetic energy depends on frequency, not intensity: K_max = hf-ϕ. Intensity affects number of emitted electrons (photon count), not their individual energies.
Wave theory couldn’t explain these points; neither special relativity nor E = mc² is involved here.

Explanation: D
Connecting an uncharged conductor A to a charged conductor B lets charge flow until both are at the same potential. Some charge moves from B to A (if B was positively charged, B loses some positive charge), so B’s charge changes. Also, the combined system has a larger effective capacitance; since V = Q/C, B’s potential changes (typically decreases in magnitude). Hence, both charge and potential of B change.

Explanation: B

Explanation: B
• Mass is the amount of matter and does not change with location → stays 10 kg on Earth and the Moon.
• Weight depends on gravity:
→ On Earth: W = mg = 10 kg × 10 m/s² = 100 N so D is false.
→ On Moon: W = 10 × 1.6 = 16 N→ not zero, so C is false.
• A is wrong because mass doesn’t scale with gravity.

Explanation: B
Beta particles (high-speed electrons) undergo multiple collisions with air molecules and are easily deflected by small electric/magnetic fields. Their small mass makes them scatter strongly, so in air they follow a zig-zag, irregular (erratic) path—not straight. Circular/elliptical paths would require a uniform magnetic field.

Explanation: D

Explanation: C
 In an isolated system (no external forces), all forces are internal and occur in equal and opposite pairs (Newton’s 3rd law). These equal and opposite forces give equal and opposite impulses, so the vector sum of momenta of all bodies doesn’t change:
d/dt (p₁+p₂+..…) = 0 ⇒ total momentum conserved.

Explanation: B
Sound has long wavelengths (e.g., ~0.34 m at 1 kHz), comparable to doorways and corridors, so it diffracts (bends) around edges, letting you hear “around corners.” Visible light has very short wavelengths (~5×10^–7 m), much smaller than such openings, so diffraction is negligible—you can’t see around the corner.

Explanation: B

Explanation: C
Net force F_net = 10−4=6 N
Using F = ma: a = F_net/m = 6/2 = 3 m/s².
Since the net force is constant, the acceleration is constant (not the speed).

Explanation: D
Electric field strength E can be expressed equivalently as
E = F/q (N/C) = V/d (V/m) = (J/C/m) = J/C.m.
So, N/C, V/m and J/C.m are the same unit.

Explanation: B
In the electromagnetic spectrum, frequency increases (and wavelength decreases) in the order
radio waves → microwaves → infrared → visible light → ultraviolet → X-rays → gamma rays.
So among the given, radio < visible < X-rays.

Explanation: D
Electric potential (voltage) is energy per unit charge:
V = W/Q ⇒1 V = 1 J/C
• A: C/A = s (since A = C⋅s^–1)→ not volt.
• B: C/J = 1/V→ inverse of volt.
• C: J/A = V⋅s→ not volt.

Explanation: B
Why: Half-life T_1/2 = 2.5 min→ in 7.5 min there are 3 half-lives, so remaining fraction is (1/2)³ = 1/8.
Let initial mass m₀. After 7.5min : m = 1/8 m₀.
Decrease = m₀ – 1/8 m₀ = 7/8 m₀ = 70μ g.
So, m₀ = 70 × 8/7 = 80 μ g.

Explanation: C
A calorimeter measures the heat (enthalpy) change during physical or chemical processes by tracking temperature changes of a known mass with known specific heat. It’s not used to determine mechanism, order, or stoichiometry directly.

Explanation: D
• Moles of NaOH initially: n = CV = 2.0 mol dm^–3 × 0.020 dm³ = 0.040 mol.
• Neutralization is 1 : 1→ need 0.040 mol HCl .
• Volume of 0.50 mol dm^–3 HCl to supply 0.040 mol :
V = n/C = 0.040/0.50 = 0.080 dm³ = 80 cm³
• Total volume at neutrality = initial 20 cm³ + added 80 cm³ = 100 cm³.

Explanation: D
• Metals like copper and mercury, and graphite conduct via free electrons (electron sea/delocalized electrons).
• Ionic compounds like sodium chloride conduct by the movement of ions, but only when molten or in aqueous solution (in solid state the ions are fixed in the lattice and can’t move).

Explanation: D
• O₂ has an O = O double bond.
• CO₂ is O = C = O with two double bonds.
• Ethene (C₂H₄) has a C = C double bond.
The others listed in other options (Cl₂, N₂, CH₄, HCl) do not contain double bonds.

Explanation: D
• Soft metal (can be cut with a knife).
• Density < water → it floats.
• Reaction with water is highly exothermic; with a low melting point (~63.5 °C), the metal melts into a silvery ball and often ignites (lilac flame).
• Calcium and magnesium are harder, denser than water (don’t float) and react much less; copper doesn’t react with water.

Explanation: B
• Chlorine hydride = HCl, which dissolves in water to form hydrochloric acid (acidic), not alkaline.
A: Carbon hydride CH₄ (methane) is a gas
C: Nitrogen hydride NH₃ (ammonia) is made by the Haber process 
D: Oxygen hydride H₂O (water) is liquid at room temperature.

Explanation: D
• Condensation polymerization (step-growth) joins monomers while eliminating small molecules (like H₂O or HCl). Phenol + formaldehyde → phenol-formaldehyde (Bakelite) with water eliminated.
• Polyethene (A), Nitrile rubber (B), Teflon (C) are addition (chain-growth) polymers formed by opening double bonds (no small molecule eliminated).

Explanation: B
• CaCl₂(aq) + Na₂CO₃(aq) → CaCO₃(s)↓ (white) + 2 NaCl(aq) → white precipitate of calcium carbonate forms.
A: Ba²⁺ + Cl^– → BaCl₂ (soluble) → no ppt.
C: CO₂ through KCl(aq) → no reaction producing an insoluble salt.
D: HCl + NH₃(aq) → NH₄Cl (soluble) → no ppt.

Explanation: C
Zinc is more reactive than copper, so it displaces Cu²⁺ from solution:
• Molecular: Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
• Ionic: Zn(s) → Zn²⁺(aq) + 2e^–  (oxidation)
Cu²⁺(aq) + 2e^– → Cu(s) (reduction).
So, zinc is oxidized and copper(II) ions are reduced to copper metal. Zinc sulfate stays in solution (it doesn’t precipitate).

Explanation: C
• Alkanes are saturated (no C=C), so they undergo free-radical substitution with halogens (e.g., Cl₂ under UV).
A: Not readily polymerized (that’s typical for alkenes).
B: Addition with bromine is a test for alkenes/alkynes, not alkanes.
D: Methane = CH₄, Butane = C₄H₁₀ → hydrogen difference is 6, not 3.

Explanation: C
• Moles of N₂ =1.4/28 = 0.05 mol.
• At STP, 1 mol occupies 22.4 dm³ → Volume = 0.05 × 22.4 = 1.12 dm³.   

Explanation: C
By Fajans’ rules, covalent character increases with higher cation charge and smaller cation size (greater polarizing power), and with larger, more polarizable anions.
• NaCl: largely ionic.
• MgCl₂: ionic with slightly more covalent character than NaCl.
• AlCl₃: significant covalent character (even dimerizes to Al₂Cl₆ in vapor).
• SiCl₄: a molecular covalent compound (no ions).
Thus, SiCl₄ has the greatest covalent character among the options.

Explanation: B
Typical bond angles: CH₄ ≈ 109.5° (tetrahedral), NH₃ ≈ 107° (trigonal pyramidal), H₂O ≈ 104.5° (bent), H₂S ≈ 92° (bent). Down group 16 (O → S), larger central atom and more diffuse lone pairs reduce repulsion, so the H–X–H angle decreases—making H₂S the smallest.

Explanation: A
A pure liquid has a sharp, characteristic boiling point. Impurities broaden/shift it. Smell and colour are subjective/unreliable, and water solubility doesn’t accurately indicate purity for esters.

Explanation: A
In the hydrogen spectrum, series are named by the lower (final) orbit:
• n=1 → Lyman (UV).
• n = 2 → Balmer (visible)  
• n = 3 → Paschen (IR)
• n = 4 → Brackett (IR)
• n = 5 → Pfund (IR)
Electrons dropping to orbit 1 give the Lyman series.

Explanation: C
A spontaneous reaction proceeds on its own in a definite direction (toward equilibrium) without continuous external work, making it unidirectional and typically irreversible under given conditions. It’s a real (observable) process, not imaginary or reversible.

Explanation: B
Greater shielding effect means inner electrons block more nuclear charge from reaching outer electrons, lowering the effective nuclear charge (Z^eff) felt by them. With weaker attraction to the nucleus, electrons are easier to remove → ionization potential decreases.

Explanation: B
• Azimuthal (ℓ) quantum number → shape of the orbital (s, p, d, f).
• Principal (n) → size/energy level.
• Magnetic (m_l) → orientation in space.
• Spin (m_s) → electron spin direction.

Explanation: C
Number of nodes in an orbital = n − 1.
For 4s: n = 4 ⇒ total nodes = 4−1 = 3.
Since l = 0 for s-orbitals, all 3 are radial nodes (no angular nodes).

Explanation: C
As you go down a group, each element has an extra electron shell (higher n) compared to the one above. This increases the average distance of valence electrons from the nucleus and increases shielding, so atomic radius increases despite the higher nuclear charge.

Explanation: D
Alkali metals are too reactive to be obtained by chemical reduction of their compounds. They’re produced by electrolysis of molten salts (e.g., Na from molten NaCl in the Down’s cell, K from molten KCl), which avoids side reactions and provides the pure metal.

Explanation: A
A True: Many d-block elements have unpaired d-electrons, so they’re paramagnetic.
B While transition metals are good conductors, that’s true of many metals and isn’t distinctive.
C Colors of transition-metal ions are mainly from d–d transitions (charge transfer can occur but isn’t the general cause).
D Melting points don’t increase directly with valence electrons; they peak mid-series due to bonding from partially filled d-orbitals.

Explanation: A
Industrial phenol is chiefly made by the cumene process:
cumene → (air oxidation) cumene hydroperoxide → (acid cleavage) phenol + acetone.
Ethyl or n-propyl benzene aren’t used for this large-scale route.

Explanation: D
The limiting reactant is consumed first and therefore limits how far the reaction can go. It determines the maximum possible (and thus minimum-determined) amount of product that can be formed. It’s not simply the one with lesser grams or volume (A/B); it’s about moles relative to the reaction stoichiometry.