Q.Choose the incorrect statement from the following:

The four listed pairs correspond to a, b, c, d respectively.

1. (a) First month – Formation of heart. ✓ Correct. "After one month of pregnancy, the embryo's heart is formed."

2. (b) Second month – Movement of foetus. ✗ Mismatched. By the end of the second month the foetus develops limbs and digits. The first movements of the foetus (together with the appearance of hair on the head) are observed during the fifth month. So pairing month 2 with foetal movement is incorrect.

3. (c) Third month – Formation of most of the major organ systems. ✓ Correct. By the end of 12 weeks (the first trimester) most of the major organ systems are formed — for example the limbs and external genital organs are well developed.

4. (d) Sixth month – Eyelids separate and eyelashes are found. ✓ Correct. By the end of 24 weeks (the second trimester) — i.e. six months — the body is covered with fine hair, the eye-lids separate, and eyelashes are formed.

The single mismatched pair is therefore the second one, labelled b.

ANSWER: A

Step 1 — The route of infection.

After entering the body, HIV first enters macrophages, where its RNA genome is copied into DNA by reverse transcriptase; this viral DNA integrates into the host cell's DNA and directs the production of virus particles. The macrophage keeps functioning as an HIV factory.

Step 2 — The key target cell.

HIV then enters helper T-lymphocytes ($T_H$ cells) — it does so because its envelope glycoprotein binds the CD4 receptor, and helper T cells are the CD4⁺ population. Inside them, the virus replicates and produces progeny viruses, which are released into the blood and go on to attack further helper T cells.

Step 3 — Why this matters clinically (the internal check).

This is exactly why AIDS is diagnosed and staged by a falling helper T-cell count: the very cell the immune system needs to coordinate its response is being destroyed as the virus multiplies inside it. The patient then becomes vulnerable to opportunistic infections (Mycobacterium, Toxoplasma, fungi).

Step 4 — Eliminate.

• (A) Memory T-lymphocytes — store immunological memory; not the CD4 target of HIV replication. ✗
• (B) Killer (cytotoxic) T-lymphocytes — these are CD8⁺, not CD4⁺. ✗
• (C) Suppressor T-lymphocytes — regulate the response; not HIV's replication site. ✗
• (D) Helper T-lymphocytes — CD4⁺, the site of HIV replication and progeny production. ✓

ANSWER: D

Step 1 — The three layers of the uterine wall

Step 2 — Why the endometrium is glandular

The endometrium is the functional, hormone-responsive lining:

• Under estrogen it proliferates (proliferative phase).
• Under progesterone from the corpus luteum its glands become secretory, storing glycogen to nourish an arriving blastocyst (secretory phase).
• It is the layer in which implantation occurs.
• If fertilisation fails, the corpus luteum degenerates, progesterone falls, and the endometrium breaks down and is shed — this is menstruation.

All of those functions depend on its glands — hence "glandular layer".

Step 3 — Eliminate the other options

• (A) Smooth muscle — that is the myometrium, the middle layer, not the endometrium. This is the trap option.
• (C) Adipose layer — fat is not a component of the uterine lining.
• (D) Cartilaginous layer — the uterus contains no cartilage at all.

ANSWER: B

Concept — Structure of the human spermatozoon (Human Reproduction). A sperm is a microscopic haploid cell made of a head, neck, middle piece and tail, all covered by a plasma membrane.

1. Head — contains an elongated haploid nucleus carrying the paternal genome. → s

2. Acrosome — a cap-like structure over the anterior part of the head, formed from the Golgi body and filled with hydrolytic enzymes (e.g. hyaluronidase) that help the sperm penetrate the zona pellucida of the ovum at fertilisation. → p

3. Middle piece — possesses numerous mitochondria arranged in a spiral sheath; these generate the ATP that powers the flagellar beat. → q

4. Tail — the longest part, a flagellum whose beating produces the motility essential for the sperm to reach the ovum. → r

Assembling the match: $1\text{-}s,\; 2\text{-}p,\; 3\text{-}q,\; 4\text{-}r$.

This is option (B). Option (C) wrongly gives motility to the acrosome, (A) puts the nucleus in the middle piece, and (D) puts mitochondria in the head.

ANSWER: B

The question asks about two specific developmental milestones: when the mother first feels the foetus move, and when hair begins to appear on the foetus’s head. These are standard landmarks in prenatal development, and they are often tested together because they occur at roughly the same time.

In human gestation, the first movements of the foetus — called "quickening" — are usually felt by the mother between the 4th and 5th month. However, the first clearly noticeable movements are most consistently reported around the 5th month (18–20 weeks). At this stage, the foetus is large enough and its muscles strong enough for the movements to be perceptible.

Simultaneously, the development of hair on the head (lanugo, which later becomes finer hair, and then true scalp hair) begins around the same period. By the end of the 5th month, fine hair is visible on the head, and this is the standard textbook milestone.

Let’s walk through the reasoning step by step.

1. First foetal movements

   The foetus starts moving much earlier (around 8–10 weeks), but these movements are too weak for the mother to feel. By the 5th month (18–20 weeks), the foetus has grown enough that its limb movements become strong and are felt by the mother. This is a classic milestone in obstetrics.

2. Appearance of hair on the head

   Hair follicles begin forming around the 3rd month, but visible hair on the scalp appears later. By the 5th month, the foetus has a fine layer of hair (lanugo) covering the body, and hair on the head becomes distinct. This is the accepted timing in standard biology curricula.

3. Checking the options

   • (A) 5th month: Matches both milestones.
   • (B) 1st month: Too early — the foetus is just an embryo, no movements felt, no hair.
   • (C) 8th month: Too late — both milestones occur much earlier.
   • (D) 4th month: Some mothers may feel faint movements, but the first definite movements and visible head hair are more reliably placed in the 5th month.

Step 1 — The definition. The NCERT chapter Organisms and Populations opens with exactly this idea: organisms that cannot tolerate a changing internal state regulate and thereby maintain homeostasis — constancy of the internal environment — even when the external conditions (temperature, salinity, pH, water availability) fluctuate.

Step 2 — Examples. Mammals and birds keep their body temperature near $37^\circ\mathrm C$ by sweating/shivering; the kidney holds blood osmolarity steady; insulin and glucagon hold blood glucose steady. Each is a homeostatic mechanism, and each works by negative feedback.

Step 3 — Why the other options are narrower or wrong.

• (A) Thermoregulation — maintenance of a constant body temperature only: one aspect of homeostasis, not the whole.
• (B) Metastasis — the spread of cancerous cells from a primary tumour to distant organs; completely unrelated.
• (C) Osmoregulation — maintenance of constant water and salt balance only: again a single component.
• (D) Homeostasis — the umbrella term for keeping the whole internal environment constant. ✓

ANSWER: D

Region a — the head.

Contains the acrosome, a cap of enzymes that digests the egg's outer layers — its function is to help the sperm penetrate the ovum.

Region b — the middle piece.

Densely packed with mitochondria, spiralled around the axial filament — its function is to provide the energy (ATP) the tail needs to keep moving.

Region c — the tail (flagellum).

The long flagellar structure whose whip-like beating is what actually drives the sperm's movement.

The key here is to recall the sequence of organogenesis — the process by which organs form and mature in the developing foetus. Each month brings a new set of milestones, and the order is logical: the most vital organs (like the heart) form first, followed by the body's structure (limbs, digits), then surface features (hair), and finally fine details (eyelid separation).

Let’s match each month to its landmark event.

1. First month (i) → (c) Heart

   By the end of the first month (week 4), the heart is the first functional organ to form and begins beating. This is the earliest major organogenesis event among the options.

2. Second month (ii) → (d) Limbs & digits

   During the second month (weeks 5–8), the limb buds appear and differentiate into arms, legs, fingers, and toes. This is when the basic body plan takes shape.

3. Fifth month (iii) → (b) Hairs on head

   By the fifth month (around week 20), lanugo (fine hair) covers the body, and scalp hair begins to grow. This is a later developmental feature.

4. Sixth month (iv) → (a) Separation of eyelids

   The eyelids fuse early in development (around week 8–10) to protect the eyes, and they separate again only later — typically during the sixth month (around week 24–26). So this is the last milestone among the four.

Thus the correct sequence is:

(i) → (c), (ii) → (d), (iii) → (b), (iv) → (a).

The life cycle of Plasmodium (the malarial parasite) alternates between a human host and a female Anopheles mosquito. The critical distinction to remember is that sexual reproduction (gametogony and fertilisation) happens only inside the mosquito, while asexual reproduction (schizogony) happens inside the human. Many students mistakenly think fertilisation occurs in human blood because they see gametocytes there — but those gametocytes are merely the precursors; they must be taken up by the mosquito to complete the sexual phase.

Let’s trace the sequence:

1. In humans (asexual cycle)

   When an infected mosquito bites a human, it injects sporozoites into the blood. These travel to the liver, invade liver cells, multiply, and then burst out as merozoites. The merozoites invade red blood cells (RBCs), where they multiply further, causing the cyclical fevers. Some merozoites, instead of continuing asexual multiplication, develop into gametocytes (male and female) inside RBCs. These gametocytes circulate in the blood but do not fuse inside the human — they are dormant until taken up by a mosquito.

2. In the mosquito (sexual cycle)

   When a female Anopheles mosquito bites an infected human, it sucks up blood containing these gametocytes. The gametocytes enter the mosquito’s stomach (also called the midgut). Here, the change in environment (lower temperature, higher pH) triggers the gametocytes to mature into gametes. The male gamete (microgamete) fertilises the female gamete (macrogamete) inside the stomach lumen — this is the fertilisation event.

3. After fertilisation

   The resulting zygote becomes a motile ookinete, which penetrates the stomach wall and forms an oocyst on the outer surface. Inside the oocyst, thousands of sporozoites develop. Eventually, the oocyst bursts, and sporozoites migrate to the salivary glands of the mosquito, ready to be injected into the next human host.

Step 1 — Locate the reaction.

After glycolysis, pyruvic acid formed in the cytoplasm enters the mitochondrial matrix. There it undergoes oxidative decarboxylation — the "link reaction" that connects glycolysis to the Krebs cycle.

Step 2 — The complete equation.

The reaction is catalysed by the pyruvate dehydrogenase complex, which requires several cofactors — notably Mg$^{2+}$ (shown over the arrow in the stem) and TPP.

Step 3 — Assign X and Y.

In the stem, the reaction is written as reactants $\rightarrow$ X $\rightarrow$ Y + CO$_2$ + NADH + H$^+$.

• X is the agent that acts on the reactants — the enzyme, pyruvate dehydrogenase.
• Y is the carbon-containing product that goes on to enter the Krebs cycle — acetyl CoA (the 2-carbon acetyl group carried on coenzyme A).

Step 4 — Eliminate the rest.

• (A) "Water, Acetyl CoA" — no water is involved as an intermediate here; the co-substrates are CoA and NAD$^+$. ✗
• (B) reverses the two: acetyl CoA is the product, not the catalyst, and pyruvate dehydrogenase is not a product. ✗
• (D) names oxaloacetic acid as the product — OAA is the 4-carbon acceptor within the Krebs cycle that condenses with acetyl CoA to form citrate; it is not produced by this step. ✗

ANSWER: C

Step 1 — Recall the adrenal gland.

Each adrenal gland has two parts: the outer cortex (secretes corticoids — glucocorticoids like cortisol, mineralocorticoids like aldosterone) and the inner medulla (secretes the catecholamines: adrenaline/epinephrine and noradrenaline/norepinephrine).

Step 2 — The emergency response.

Under stress, emergency or excitement, the sympathetic nervous system stimulates the adrenal medulla. The catecholamines released produce a coordinated set of effects that prepare the body to face or escape danger:

• increased heart rate, force of contraction and cardiac output
• increased rate of respiration and dilation of bronchioles
• glycogenolysis and lipolysis, raising blood glucose and free fatty acids to fuel muscles
• pupillary dilation, piloerection, sweating

Because of exactly this role they are called the emergency hormones or the hormones of Fight, Flight and Fright.

Why the others are wrong: Thyroxine (thyroid) regulates basal metabolic rate and development; oxytocin (posterior pituitary) causes uterine contraction and milk ejection; melatonin (pineal) regulates the sleep–wake rhythm; gastrin and secretin are gastrointestinal hormones controlling gastric acid and pancreatic bicarbonate secretion. None of these is an emergency hormone.

ANSWER: C