NEET Zoology - MCQ Practice Questions

Understanding:

We need to identify which disease is diagnosed using the Widal test.

Step 1: Identify the Widal test

The Widal test is a serological agglutination test that detects antibodies (agglutinins) in the patient's serum against Salmonella typhi antigens — specifically the O (somatic) and H (flagellar) antigens. A significant rise in titre confirms typhoid fever.

Step 2: Eliminate other options

Malaria is diagnosed by peripheral blood smear microscopy, rapid antigen (RDT) tests, or PCR.

Cholera is diagnosed by stool culture and dark-field microscopy for Vibrio cholerae.

Amoebiasis is diagnosed by stool examination for Entamoeba histolytica cysts or trophozoites.

Step 3: Confirm

The Widal test is exclusively used for diagnosing typhoid (enteric fever) caused by Salmonella typhi.

Answer:

The Widal test is used for the diagnosis of typhoid fever.

Quick Tip:

The Widal test is named after French physician Georges-Fernand Widal. An agglutination titre of 1:160 or above for O antigen is considered significant in most clinical settings — worth remembering for applied MCQs.

Understanding:

We need to identify the drug that binds to opioid receptors, produces analgesia and euphoria, and is derived from Papaver somniferum (opium poppy).

Step 1: Identify the source plant and mechanism for each option

Cocaine — derived from Erythroxylum coca; it inhibits reuptake of dopamine, serotonin, and norepinephrine; classified as a stimulant, not an opioid.

Morphine — derived from the latex of Papaver somniferum; it directly binds to opioid receptors (mu, kappa, delta) in the CNS, producing powerful analgesia and euphoria; classified as an opioid narcotic analgesic.

Cannabis — derived from Cannabis sativa; acts on cannabinoid receptors (CB1, CB2), not opioid receptors.

LSD (Lysergic acid diethylamide) — a synthetic hallucinogen derived from ergot fungus; acts primarily on serotonin receptors.

Step 2: Confirm

Morphine satisfies all three criteria: opioid receptor binding, analgesia and euphoria, and derivation from Papaver somniferum latex.

Answer:

Morphine is the drug derived from Papaver somniferum latex that binds to opioid receptors to produce analgesia and euphoria.

Quick Tip:

All three — morphine, heroin (diacetylmorphine), and codeine — are opioids from Papaver somniferum. Heroin is synthesised by acetylation of morphine and is more potent due to faster CNS penetration.

Understanding:

We need to identify the primary target cell of HIV that leads to progressive collapse of the immune system in AIDS.

Step 1: Mechanism of HIV infection

HIV (Human Immunodeficiency Virus) is a retrovirus. Its surface glycoprotein gp120 binds specifically to the CD4 receptor present on the surface of helper T lymphocytes (T-helper cells / CD4+ T cells). The co-receptors CCR5 or CXCR4 also facilitate entry.

Step 2: Consequence of CD4+ T cell depletion

Helper T cells are the master regulators of adaptive immunity. They stimulate B cells to produce antibodies (humoral arm) and activate cytotoxic T cells (cell-mediated arm). Progressive destruction of CD4+ T cells by HIV reduces their count from the normal range of 500–1500 cells/µL. When the count falls below 200 cells/µL, the patient is diagnosed with AIDS and becomes susceptible to opportunistic infections.

Step 3: Eliminate incorrect options

CD8+ T cells (cytotoxic) and NK cells are not the primary targets of HIV.

B lymphocytes lack CD4 receptors and are not directly infected.

Answer:

HIV preferentially infects helper T cells (CD4+ T cells), destroying them and leading to progressive immunodeficiency (AIDS).

Quick Tip:

The CD4+ T cell count is the key diagnostic and prognostic marker in HIV/AIDS. A count below 200 cells/µL defines AIDS irrespective of symptoms.

Understanding:

We need to identify the correct description of the role of memory cells in the secondary immune response.

Step 1: Formation of memory cells

During the primary immune response (first exposure to an antigen), some activated B and T lymphocytes differentiate into long-lived memory cells instead of effector cells. These memory cells persist in the body for years or even a lifetime.

Step 2: Role in secondary immune response

On second exposure to the same antigen, memory cells are rapidly activated and proliferate quickly (clonal expansion). This results in:

This is called an anamnestic (secondary) response and forms the immunological basis of vaccination.

Step 3: Eliminate incorrect options

Memory cells are produced during the primary immune response, not only the secondary one.

Memory B cells lead to antibody production; they do not directly kill cells (that is the role of cytotoxic T cells).

Memory cells are long-lived — they can persist for decades, not just a few days.

Answer:

Memory cells respond rapidly to a second antigenic exposure, producing a faster and stronger antibody response than the primary immune response.

Quick Tip:

The secondary immune response is the scientific basis of booster doses in vaccination — the booster rapidly reactivates memory cells to produce high antibody titres.

Understanding:

We need to identify the correct mode of transmission of Ascaris lumbricoides, the roundworm causing ascariasis.

Step 1: Life cycle of Ascaris lumbricoides

Adult worms live in the small intestine of humans and produce eggs excreted in faeces. In soil, these eggs develop into infective embryonated eggs (containing the second-stage larva) over 2–3 weeks under warm, moist conditions. Infection occurs when a human ingests these embryonated eggs through contaminated food or water.

Step 2: After ingestion

Once swallowed, larvae hatch in the small intestine, penetrate the intestinal wall, travel through the liver via portal blood, then to the lungs via pulmonary circulation (causing Löffler's syndrome), ascend the trachea, are swallowed, and finally mature into adult worms in the small intestine.

Step 3: Eliminate other options

Anopheles mosquito bite transmits Plasmodium (malaria).

Direct skin penetration is characteristic of Hookworm (Ancylostoma duodenale / Necator americanus), not Ascaris.

Ascaris is not sexually transmitted.

Answer:

Ascaris lumbricoides infects humans through ingestion of embryonated eggs from contaminated food and water.

Quick Tip:

Ascaris has the largest egg output among intestinal parasites — a single female can lay up to 200,000 eggs per day. This explains the widespread prevalence of ascariasis in areas with poor sanitation.

Understanding:

We need to identify the correct mechanism by which heroin produces its addictive effects in the nervous system.

Step 1: Chemical nature of heroin

Heroin (diacetylmorphine) is synthesised by acetylating two hydroxyl groups of morphine. This chemical modification makes heroin more lipid-soluble than morphine, allowing it to cross the blood-brain barrier much faster.

Step 2: Mechanism of action

Once inside the CNS, heroin is rapidly de-acetylated (hydrolysed) back to morphine. Morphine then binds to endogenous opioid receptors (mu, kappa, delta receptors) in the brain and spinal cord. This mimics the action of endogenous opioids (endorphins, enkephalins), causing:

Step 3: Basis of addiction

Repeat activation of the dopaminergic reward pathway by opioid receptor stimulation leads to neuroadaptation, tolerance, and physical dependence — the hallmarks of addiction.

Step 4: Eliminate incorrect options

Adrenergic receptor stimulation is the mechanism of cocaine and amphetamines, not heroin.

Blocking ACh at the neuromuscular junction is the mechanism of neurotoxins like botulinum or curare.

Serotonin antagonism and hallucinations describe LSD, not heroin.

Answer:

Heroin is rapidly converted to morphine in the body, which then binds to opioid receptors in the CNS, producing euphoria, analgesia, and CNS depression leading to addiction.

Quick Tip:

Heroin's higher lipid solubility compared to morphine explains its faster onset and greater abuse potential — it enters the brain quicker, causing a more intense initial rush.