NEET Zoology - MCQ Practice Questions
Zoology in NEET is memory heavy but not memory only, because the diagram based and assertion reason questions need understanding. This set runs through human physiology, animal kingdom and classification, structural organisation, genetics and evolution, biotechnology, and ecology. Explanations point to the NCERT line the question came from, so revision stays anchored to the source text.
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Ascaris lumbricoides, the causative agent of ascariasis, infects humans primarily through which of the following routes?
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.
Which of the following correctly describes the mechanism by which heroin (diacetylmorphine) acts as an addictive drug in the nervous system?
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.
Which of the following pathogens is responsible for causing Kala-azar (Visceral Leishmaniasis) in humans, and what is its primary vector?
Understanding:
We need to identify the causative agent of Kala-azar (Visceral Leishmaniasis) and its vector.
Step 1: Identify the pathogen
Kala-azar is caused by Leishmania donovani, a protozoan parasite. It is an intracellular parasite that resides within macrophages of the liver, spleen, and bone marrow, leading to enlargement of these organs (hepatosplenomegaly).
Step 2: Identify the vector
The vector for Leishmania donovani is the female Phlebotomus sandfly. The sandfly injects the promastigote form of the parasite during a blood meal. Inside the human host, the parasite transforms into the amastigote form.
Step 3: Eliminate incorrect options
Trypanosoma cruzi causes Chagas disease and is transmitted by the Reduviid (kissing) bug, not the Tsetse fly (which transmits African sleeping sickness). Plasmodium vivax causes malaria, transmitted by the female Anopheles mosquito. Anopheles is not the vector for Leishmaniasis.
Answer:
Kala-azar is caused by Leishmania donovani, transmitted by the female Phlebotomus sandfly.
Quick Tip:
Remember the triad: Kala-azar → Leishmania donovani → Phlebotomus sandfly. "Kala-azar" means "black fever" in Hindi, reflecting the darkening of skin seen in affected individuals.
In active immunisation, which of the following best describes the basis of its long-term protective effect?
Understanding:
We need to understand why active immunisation provides long-term protection.
Step 1: Define active immunisation
Active immunisation involves introducing an antigen (via a vaccine — attenuated, killed pathogen, or subunit) into the body. The immune system mounts a primary immune response, producing effector cells and antibodies.
Step 2: Role of memory cells
Crucially, this primary response also generates long-lived memory B cells and memory T cells. These cells persist in the body for years or decades. Upon re-exposure to the same pathogen, these memory cells rapidly proliferate and differentiate, producing a faster, stronger, and more sustained secondary immune response — often eliminating the pathogen before disease develops.
Step 3: Eliminate incorrect options
Pre-formed antibody injection describes passive immunisation (e.g., antivenom, antitoxin), which provides immediate but short-lived protection. Transfer of antibodies from donor to recipient is also passive immunisation. Innate immune cells do not provide the antigen-specific memory that underlies long-term protection.
Answer:
Active immunisation works by generating immunological memory through memory B and T cells, which ensure rapid and robust protection upon future exposure to the pathogen.
Quick Tip:
Active immunisation = the host actively produces its own antibodies and memory cells. Passive immunisation = antibodies are given externally — fast but temporary.
The drug cocaine exerts its stimulant effect primarily by which of the following mechanisms?
Understanding:
We need to identify the mechanism of action of cocaine as a stimulant drug.
Step 1: Cocaine's pharmacological action
Cocaine is a powerful central nervous system stimulant. It blocks the reuptake transporters for dopamine, norepinephrine, and serotonin at presynaptic nerve terminals. Normally, after neurotransmitter release, these molecules are transported back into the presynaptic neuron (reuptake). Cocaine prevents this process.
Step 2: Consequence of reuptake blockade
With reuptake blocked, dopamine (and other monoamines) accumulate in the synaptic cleft, overstimulating postsynaptic receptors. The flood of dopamine in the reward pathway (nucleus accumbens) produces intense euphoria and is responsible for cocaine's addictive properties.
Step 3: Eliminate incorrect options
Opioid receptor stimulation is the mechanism of morphine and heroin, not cocaine. GABA enhancement is the mechanism of benzodiazepines and alcohol — GABA is inhibitory, producing sedation rather than stimulation. Nicotinic acetylcholine receptor binding describes nicotine's mechanism.
Answer:
Cocaine acts by blocking the reuptake of dopamine, norepinephrine, and serotonin, causing their accumulation in the synapse and producing stimulant and euphoric effects.
Quick Tip:
Cocaine is a reuptake inhibitor — it does NOT directly stimulate receptors. This distinction is commonly tested in NEET.
Rheumatoid arthritis is an example of which of the following types of disease?
Understanding:
We need to classify rheumatoid arthritis correctly in terms of its immunological basis.
Step 1: Define autoimmune disease
Autoimmune diseases arise when the immune system fails to distinguish self from non-self and mounts an immune response against the body's own tissues. This loss of self-tolerance leads to chronic inflammation and tissue damage.
Step 2: Rheumatoid arthritis as an autoimmune disease
In rheumatoid arthritis, the immune system (particularly T cells and autoantibodies called rheumatoid factor) attacks the synovial membranes lining the joints. This causes chronic inflammation, pain, swelling, and progressive joint destruction.
Step 3: Eliminate incorrect options
IgE-mediated allergic diseases include asthma, hay fever, and anaphylaxis — not rheumatoid arthritis. Streptococcus pyogenes causes pharyngitis and can lead to acute rheumatic fever, which is different from rheumatoid arthritis. Immunodeficiency involves loss of immune function, whereas in autoimmune disease, immunity is misdirected, not absent.
Answer:
Rheumatoid arthritis is a classic autoimmune disease in which the immune system attacks the body's own joint tissues, causing chronic inflammatory damage.
Quick Tip:
Other autoimmune diseases to remember for NEET: Type 1 diabetes mellitus (attack on beta cells), multiple sclerosis (attack on myelin), and systemic lupus erythematosus (SLE).
During the life cycle of Plasmodium, which stage of the parasite is injected into the human bloodstream by the infected female Anopheles mosquito?
Understanding:
We need to identify the correct infective stage of Plasmodium that enters the human body via a mosquito bite.
Step 1: Plasmodium life cycle overview
The life cycle of Plasmodium involves two hosts — the female Anopheles mosquito (definitive host, where sexual reproduction occurs) and humans (intermediate host, where asexual reproduction occurs).
Step 2: Stage injected into humans
When an infected female Anopheles mosquito bites a human, it injects sporozoites from its salivary glands into the bloodstream. Sporozoites are the infective stage for humans. They travel to the liver and invade hepatocytes, where they undergo schizogony to form merozoites.
Step 3: Other stages and their roles
Merozoites are released from liver cells and infect red blood cells (RBCs) — they are NOT the stage injected by the mosquito. Gametocytes are the sexual stage found in human blood — they are taken up by the mosquito during a blood meal. Trophozoites are the feeding stage inside RBCs.
Answer:
The sporozoite is the stage injected into the human bloodstream by the infected female Anopheles mosquito.
Quick Tip:
Memory aid: Sporozoites → Liver (Schizogony) → Merozoites → RBCs → Gametocytes → Mosquito. The mosquito injects sporozoites; the mosquito ingests gametocytes.
Which of the following correctly describes the difference between benign and malignant tumours?
Understanding:
We need to distinguish between benign and malignant tumours based on their biological behaviour.
Step 1: Characteristics of benign tumours
Benign tumours are masses of cells that proliferate abnormally but remain confined to the tissue of origin. They are encapsulated, do not invade adjacent tissues, do not metastasise (spread to distant sites), and are generally not life-threatening, though they can cause problems by pressing on nearby structures.
Step 2: Characteristics of malignant tumours
Malignant tumours (cancers) are characterised by uncontrolled proliferation, loss of contact inhibition, invasion of surrounding normal tissues, and metastasis — the ability to spread to distant organs via the bloodstream or lymphatic system. Metastasis is the key property that makes cancer potentially fatal.
Step 3: Eliminate incorrect options
Neither tumour type is exclusively caused by viruses. Benign tumours are generally slow-growing and less dangerous. There is no organ restriction for where benign tumours form.
Answer:
Benign tumours are localised and non-invasive, while malignant tumours invade surrounding tissues and can metastasise to distant organs through blood or lymph.
Quick Tip:
The word malignant comes from Latin meaning "evil-born" — the hallmark property that distinguishes it from benign tumours is metastasis. Contact inhibition is lost in cancer cells.
Which of the following statements correctly describes the role of interferons in the body's defence against viral infections?
Understanding:
We need to understand the mechanism of action of interferons in viral defence.
Step 1: What are interferons?
Interferons (IFNs) are a family of signalling proteins (cytokines) that form a key part of the innate immune response to viral infections. They are not antibodies and are not specific to a single virus.
Step 2: How do interferons work?
When a cell is infected by a virus, it synthesises and secretes interferons. These interferons bind to receptors on neighbouring uninfected cells and trigger the expression of antiviral genes (such as those encoding protein kinases and endonucleases). This creates an "antiviral state" in the neighbouring cells, limiting the spread of the virus. Interferons also enhance natural killer (NK) cell activity and promote adaptive immune responses.
Step 3: Eliminate incorrect options
Interferons are not antibodies — antibodies are produced by plasma cells (differentiated B cells) and are antigen-specific. Interferons are not enzymes that directly lyse viruses. While NK cells respond to interferon signals, interferons themselves are produced by infected cells (including fibroblasts, leukocytes, and T cells) and act before or alongside adaptive immunity.
Answer:
Interferons are antiviral cytokines secreted by infected cells that warn and protect neighbouring cells by inducing an antiviral state, limiting viral spread.
Quick Tip:
Interferons do not directly kill viruses — they "interfere" with viral replication by making neighbouring cells inhospitable to viral multiplication. This is why they are part of the non-specific (innate) immune response.
A patient presents with high fever, chills, headache, and jaundice. Blood examination reveals parasitised red blood cells. The patient reports visiting a forest area. Which of the following is the most likely causative species of Plasmodium, and why is it considered the most dangerous?
Understanding:
We need to identify the most dangerous species of Plasmodium and explain why, in the context of a patient with severe malaria symptoms.
Step 1: Species of Plasmodium infecting humans
Four species of Plasmodium infect humans: P. vivax, P. falciparum, P. malariae, and P. ovale. Among these, P. falciparum causes malignant tertian malaria and is the most lethal.
Step 2: Why P. falciparum is the most dangerous
Unlike other species, P. falciparum infects RBCs of ALL ages (not just young or old RBCs), leading to a massive parasitaemia and severe haemolytic anaemia. Infected RBCs become sticky and form rosettes, clogging capillaries in vital organs. When this occlusion occurs in cerebral blood vessels, it causes cerebral malaria — a medical emergency with high mortality. P. falciparum also causes blackwater fever (haemoglobinuria due to massive haemolysis).
Step 3: Eliminate incorrect options
P. vivax causes relapse through hypnozoites but is rarely fatal. P. malariae causes quartan fever (72-hour cycle) and is milder. P. ovale is the rarest and is not resistant to all antimalarials.
Answer:
Plasmodium falciparum is the most dangerous species because it infects RBCs of all ages, causes severe anaemia, and can lead to cerebral malaria through capillary occlusion.
Quick Tip:
Remember the fever patterns: P. vivax/ovale = tertian (48 hours, benign); P. falciparum = malignant tertian (48 hours, dangerous); P. malariae = quartan (72 hours).
Which of the following statements regarding the ELISA (Enzyme-Linked Immunosorbent Assay) test is correct in the context of HIV/AIDS diagnosis?
Understanding:
We need to understand what ELISA detects in HIV diagnosis and how a positive result is confirmed.
Step 1: Principle of ELISA in HIV diagnosis
ELISA (Enzyme-Linked Immunosorbent Assay) is a serological test that detects the presence of antibodies against HIV antigens in the patient's serum or plasma. HIV antigens are coated on a plate; if the patient's serum contains anti-HIV antibodies, they bind to these antigens. An enzyme-linked secondary antibody is then added, and a colour change (upon substrate addition) indicates a positive result.
Step 2: Confirmatory test
Because ELISA can occasionally give false positives, a reactive (positive) ELISA result must be confirmed by the Western Blot test, which is more specific and identifies antibodies against individual HIV proteins (e.g., gp120, gp41, p24).
Step 3: Eliminate incorrect options
DNA amplification from blood cells describes PCR (Polymerase Chain Reaction), not ELISA. CD4+ T cell count is measured by flow cytometry (CD4 count), not ELISA. ELISA is widely used for both blood screening and clinical diagnosis of HIV.
Answer:
ELISA detects anti-HIV antibodies in the patient's serum, and a reactive result is confirmed by the more specific Western Blot test.
Quick Tip:
ELISA = screening test (sensitive); Western Blot = confirmatory test (specific). For NEET, remember this hierarchy: ELISA first, then Western Blot to confirm HIV infection.
Which of the following pairs correctly matches a disease with its causative agent AND the correct category of that agent?
Understanding:
We need to match a disease with its correct causative agent and the correct biological category of that agent.
Step 1: Evaluate each option
Option A: Typhoid is caused by Salmonella typhi — this pathogen is a bacterium, NOT a virus. So the category is wrong.
Step 2:
Option B: Pneumonia can be caused by Streptococcus pneumoniae, which is indeed a bacterium (Gram-positive diplococcus). This match is completely correct — the disease, causative agent, and category all align.
Step 3:
Option C: Ringworm is caused by fungi (dermatophytes such as Trichophyton and Microsporum), NOT by Taenia solium. Taenia solium is a tapeworm (helminth) that causes taeniasis/cysticercosis. So both the agent and category are wrong.
Step 4:
Option D: Common cold is caused by Rhinovirus (a virus), NOT by Plasmodium vivax. Plasmodium vivax is a protozoan causing malaria. Both the agent and category are wrong.
Answer:
Pneumonia caused by Streptococcus pneumoniae is correctly categorised as a bacterial disease.
Quick Tip:
Ringworm is a common NEET trap — despite the name, it is NOT caused by a worm. It is a fungal skin infection caused by dermatophytes. Always remember: Ringworm = Fungi.
Which of the following correctly describes the process of oogenesis in humans, with respect to the stage at which meiosis is arrested before ovulation?
Understanding:
We need to identify the correct stage at which the primary oocyte arrests during oogenesis in humans before ovulation occurs.
Step 1: Tracing the stages of oogenesis
Oogenesis begins during foetal development. Oogonia multiply by mitosis and then enter meiosis I to become primary oocytes. These primary oocytes do NOT complete meiosis I; instead, they are arrested at Prophase I (specifically at the diplotene stage of Prophase I) and remain in this suspended state until puberty.
Step 2: Events at ovulation
Just before ovulation, the primary oocyte resumes meiosis I and completes it, producing a secondary oocyte and the first polar body. The secondary oocyte then begins meiosis II but arrests at Metaphase II. It is the secondary oocyte (arrested at Metaphase II) that is actually released at ovulation.
Step 3: Identifying the correct answer
The question specifically asks about the stage of arrest BEFORE ovulation. Before ovulation, it is the primary oocyte that is arrested, and it is held at Prophase I of meiosis I. This arrest can last for years (even decades) in the ovary until the oocyte is stimulated to mature.
Answer:
The primary oocyte is arrested at Prophase I of meiosis I before ovulation.
Quick Tip:
Remember the two arrest points: Primary oocyte arrests at Prophase I (before ovulation); Secondary oocyte arrests at Metaphase II (released at ovulation, completes meiosis II only if fertilised).
In the human male reproductive system, the correct sequence of structures through which sperms travel from the site of production to the urethra is:
Understanding:
We need to identify the correct sequential pathway that spermatozoa follow from the seminiferous tubules to the urethra in the human male reproductive system.
Step 1: Identifying the origin
Sperms are produced in the seminiferous tubules located inside the testes.
Step 2: Pathway through the testis
From the seminiferous tubules, sperms move into the rete testis, a network of tubules within the testis. From the rete testis, sperms pass through the vasa efferentia (efferent ductules), which carry them to the epididymis.
Step 3: Pathway from testis to urethra
The epididymis is the site where sperms mature and are stored. From the epididymis, sperms enter the vas deferens (ductus deferens). The vas deferens ascends into the abdomen, loops around the urinary bladder, and joins with the duct of the seminal vesicle to form the ejaculatory duct. The ejaculatory duct opens into the urethra, which carries sperms out of the body.
Step 4: Complete correct sequence
Seminiferous tubules → Rete testis → Vasa efferentia → Epididymis → Vas deferens → Ejaculatory duct → Urethra
Answer:
The correct pathway of sperm transport is from seminiferous tubules through the rete testis, vasa efferentia, epididymis, vas deferens, and ejaculatory duct to the urethra.
Quick Tip:
A common mistake is placing the epididymis before the rete testis. Remember: rete testis → vasa efferentia → epididymis (not the reverse).
Which of the following statements correctly describes the role of Sertoli cells in the human testis?
Understanding:
We need to identify the correct description of the functions and regulation of Sertoli cells (also called nurse cells or sustentacular cells) in the human testis.
Step 1: Location and identity of Sertoli cells
Sertoli cells are large, non-dividing cells found lining the seminiferous tubules. They are distinct from Leydig cells (interstitial cells), which are located in the interstitial spaces between the tubules.
Step 2: Functions of Sertoli cells
Sertoli cells provide structural support and nutrition to developing spermatogenic cells (from spermatogonia to spermatozoa). They form the blood-testis barrier along with other cells. They also secrete inhibin, which provides negative feedback on FSH secretion, and androgen-binding protein (ABP).
Step 3: Hormonal regulation of Sertoli cells
Sertoli cells are primarily regulated by Follicle Stimulating Hormone (FSH) secreted by the anterior pituitary gland. FSH stimulates Sertoli cells to support spermatogenesis.
Step 4: Distinguishing Sertoli cells from Leydig cells
Leydig cells (not Sertoli cells) secrete testosterone and are regulated by Luteinising Hormone (LH). Sertoli cells do NOT secrete LH; LH is a pituitary hormone. Sertoli cells secrete inhibin, but do not directly stimulate mitosis of spermatogonia (FSH-driven Sertoli support does so indirectly).
Answer:
Sertoli cells provide nutrition to developing sperms and are regulated by FSH from the anterior pituitary.
Quick Tip:
Memory aid: FSH → Sertoli cells (sustenance/support); LH → Leydig cells (testosterone). The first letters F-S and L-L help link the hormone to its target cell.
Which of the following correctly describes the placenta and its role as an endocrine structure during human pregnancy?
Understanding:
We need to identify the correct statement about the placenta's role as an endocrine organ during human pregnancy.
Step 1: Structure of the placenta
The placenta is a temporary endocrine organ formed by contributions from both the trophoblast (foetal tissue) and the endometrium (maternal tissue). It is not formed entirely from the endometrium.
Step 2: Hormones secreted by the placenta
The placenta secretes several hormones: human chorionic gonadotropin (hCG), progesterone, oestrogens, and human placental lactogen (hPL). hCG is produced very early in pregnancy and is the hormone detected in pregnancy tests.
Step 3: Role of hCG
During early pregnancy, hCG is secreted by the trophoblast and later by the placenta. Its primary role is to maintain the corpus luteum in the ovary, preventing it from degenerating. The corpus luteum, in turn, continues to secrete progesterone and oestrogen, which are essential for maintaining the uterine lining (endometrium) and supporting the pregnancy until the placenta takes over this function.
Step 4: Evaluating the other options
The placenta does secrete progesterone and oestrogens AND hCG (especially in early pregnancy), so saying it does NOT secrete hCG after the first trimester is partially true but the key role of hCG maintaining the corpus luteum is the critical fact being tested. Prolactin is secreted by the anterior pituitary, not the placenta. The placenta has contributions from both foetal and maternal tissue.
Answer:
The placenta secretes hCG, which maintains the corpus luteum during early pregnancy, ensuring continued progesterone and oestrogen production.
Quick Tip:
hCG is the hormone detected in home pregnancy tests. Its presence in urine/blood within days of implantation makes it the earliest hormonal marker of pregnancy.
In humans, which of the following contraceptive methods works primarily by preventing ovulation through inhibition of the hypothalamo-pituitary axis?
Understanding:
We need to identify the contraceptive method that works by preventing ovulation through suppression of the hypothalamo-pituitary axis.
Step 1: Mechanism of combined oral contraceptive pills
Combined oral contraceptive pills contain synthetic oestrogen and progesterone (or progesterone alone in mini-pills). When taken regularly, these hormones exert negative feedback on the hypothalamus and anterior pituitary gland. This suppresses the secretion of GnRH from the hypothalamus and subsequently reduces the secretion of FSH and LH from the anterior pituitary. Without the LH surge, ovulation does not occur. This is the primary mechanism of action.
Step 2: Mechanism of Copper-T
Copper-T is an intrauterine device (IUD) that works primarily by creating a hostile environment in the uterus (copper ions are spermicidal and affect sperm motility and viability) and by preventing implantation. It does NOT prevent ovulation.
Step 3: Mechanism of condom
Condoms act as physical barriers, preventing sperms from reaching the ovum. They have no effect on the hormonal axis and do not prevent ovulation.
Step 4: Mechanism of tubectomy
Tubectomy is a surgical procedure in which the fallopian tubes are cut and ligated (tied). This prevents the ovum from reaching the uterus and prevents fertilisation. It does not affect the hormonal axis or ovulation.
Answer:
Combined oral contraceptive pills prevent ovulation by suppressing the hypothalamo-pituitary axis through negative feedback of synthetic oestrogen and progesterone.
Quick Tip:
A useful classification: barrier methods (condom, diaphragm) → prevent sperm-egg meeting; hormonal methods (pills) → prevent ovulation; IUDs (Copper-T) → prevent fertilisation/implantation; surgical methods (vasectomy/tubectomy) → permanent blockage.