Infectious Diseases Practice Questions

Q: What is the incubation period of Diphtheria?

2-6 days. **Explanation:** **1. Why Option A is Correct:** The incubation period of Diphtheria, caused by *Corynebacterium diphtheriae*, is typically **2 to 6 days**. This short duration reflects the rapid colonization of the upper respiratory tract (or skin) and the subsequent production of the potent diphtheria exotoxin. The toxin acts locally to cause tissue necrosis and the formation of the characteristic "pseudomembrane," while systemic absorption can lead to myocarditis and polyneuritis. **2. Why Other Options are Incorrect:** * **Option B (2-6 hours):** This is characteristic of pre-formed bacterial toxins in food poisoning, such as *Staphylococcus aureus* or *Bacillus cereus* (emetic type). * **Option C (2-6 weeks):** This is too long for Diphtheria. Such durations are typical for diseases like Hepatitis A, Typhoid, or the early stages of Syphilis. * **Option D (2-6 months):** This prolonged period is seen in slow-growing infections like Hepatitis B, Rabies (variable), or certain filarial infections. **3. High-Yield Clinical Pearls for NEET-PG:** * **Causative Agent:** *Corynebacterium diphtheriae* (Gram-positive, club-shaped, Chinese-letter pattern). * **Schick Test:** Used to demonstrate susceptibility/immunity to Diphtheria. * **Culture Media:** **Löffler's serum slope** (rapid growth) and **Potassium Tellurite agar** (black colonies). * **Case Definition:** Presence of a greyish-white adherent pseudomembrane; attempting to remove it causes bleeding. * **Public Health:** The period of communicability is usually 14–28 days without antibiotics. Cases are considered non-infectious after **two consecutive negative cultures** taken 24 hours apart.

Q: Which of the following statements regarding poliomyelitis is true?

It commonly spreads by the fecal-oral route.. ### Explanation **1. Why Option C is Correct:** Poliomyelitis is primarily an enteric infection. In areas with poor sanitation and hygiene (common in developing countries), the **fecal-oral route** is the most important mode of transmission. The virus multiplies in the Peyer's patches of the ileum and is excreted in the feces for several weeks. In the early stages of infection and in areas with high hygiene standards, droplet infection (respiratory route) can also occur, but the fecal-oral route remains the predominant epidemiological driver. **2. Why Other Options are Incorrect:** * **Option A:** Poliovirus is **heat-sensitive**. It is readily inactivated by **pasteurization** (62.5°C for 30 minutes), as well as by boiling and formaldehyde. * **Option B:** The ratio of clinical to subclinical cases varies by age. For every clinical case, there are roughly **1000 subclinical cases in children**, but only about **75 to 100 subclinical cases in adults**. Adults are more prone to developing paralytic disease if infected. * **Option D:** **Type 1 poliovirus** is the most common cause of paralytic poliomyelitis and is responsible for most outbreaks. Type 2 was the first to be eradicated globally (2015), followed by Type 3 (2019). **3. High-Yield Clinical Pearls for NEET-PG:** * **Reservoir:** Man is the only known reservoir (no animal carrier). * **Infectivity:** Most infectious during the late incubation period and the first week of clinical illness. * **Immunity:** Type-specific and lifelong. * **Vaccine:** **Salk (IPV)** induces humoral immunity (IgG); **Sabin (OPV)** induces both humoral and local intestinal immunity (IgA), preventing the spread of wild virus. * **VAPP vs VDPV:** Vaccine-Associated Paralytic Polio (VAPP) occurs in the vaccine recipient; Vaccine-Derived Poliovirus (VDPV) is due to the circulation of the mutated vaccine virus in the community.

Q: Which of the following is true about the Anopheles mosquito?

Larva lacks a siphon tube. ### Explanation The correct answer is **B. Larva lacks a siphon tube.** In epidemiology and medical entomology, distinguishing between mosquito genera is a high-yield topic for NEET-PG. The **Anopheles** mosquito larva is unique because it lacks a respiratory siphon tube. Instead, it possesses **palmate hairs** on its abdominal segments, which allow it to lie **horizontally** (parallel) to the water surface to breathe through respiratory spiracles. #### Analysis of Options: * **Option A:** Anopheles typically breeds in **clean, fresh standing water** (e.g., rainwater, pools, margins of slow-moving streams). Breeding in **artificial containers** (like flower pots or discarded tires) is a characteristic feature of the *Aedes* mosquito. * **Option C:** The **"Tiger mosquito"** refers to ***Aedes albopictus***, named for the distinct white stripes on its body and legs. * **Option D:** The **"Nuisance mosquito"** refers to the ***Culex*** mosquito. It is so named because it is a persistent biter, often found in polluted water and urban sewage, causing significant irritation. #### High-Yield Clinical Pearls for NEET-PG: * **Resting Posture:** Adult Anopheles sit at an **angle (45°)** to the surface, whereas *Culex* and *Aedes* sit parallel. * **Eggs:** Anopheles eggs are boat-shaped and possess **lateral floats**. * **Disease Vector:** Anopheles is the primary vector for **Malaria**. * **Feeding Habit:** They are primarily **nocturnal** (night-biters), unlike *Aedes*, which is a day-biter. * **Control:** The drug of choice for biological control of Anopheles larvae is the **Gambusia affinis** (larvivorous fish).

Q: A 5-year-old child presents with rashes all over the body and was diagnosed with chickenpox. What feature helps differentiate chickenpox rash from smallpox rash?

Pleomorphic rash. ### Explanation The correct answer is **B. Pleomorphic rash**. **1. Why Pleomorphic Rash is Correct:** Chickenpox (caused by the Varicella-Zoster virus) is characterized by **pleomorphism**, meaning that different stages of the rash (papules, vesicles, and crusts/scabs) are visible simultaneously in the same anatomical area. This occurs because the rash appears in successive "crops" over several days. In contrast, smallpox is **non-pleomorphic**; all lesions in a specific area evolve at the same rate and appear identical in stage. **2. Analysis of Incorrect Options:** * **A. Centrifugal distribution:** This is a hallmark of **Smallpox**, where the rash is more dense on the face and distal extremities. Chickenpox follows a **centripetal** distribution, being most dense on the trunk and sparse on the extremities. * **C. Non-pleomorphic rash:** This is characteristic of **Smallpox**, where all lesions are at the same stage of development. * **D. Palms and soles are involved:** This is a classic feature of **Smallpox**. In Chickenpox, the palms and soles are typically **spared**. **3. High-Yield NEET-PG Clinical Pearls:** * **Dew-drop on a rose petal:** Classic description of the thin-walled varicella vesicle on an erythematous base. * **Rapid Evolution:** Chickenpox lesions evolve rapidly (within 24 hours), whereas smallpox lesions evolve slowly. * **Incubation Period:** Chickenpox is typically 14–16 days (Range: 10–21 days). * **Infectivity:** A patient is infectious from 1–2 days *before* the rash appears until all vesicles have crusted over (usually 6 days after onset). * **Scabs:** In chickenpox, scabs fall off quickly (within 14 days) and do not contain the virus, unlike smallpox scabs which are highly infectious.

Q: Which arthropod possesses four pairs of legs?

Arachnida. **Explanation:** The classification of arthropods is a high-yield topic in Medical Entomology. The correct answer is **Arachnida** because members of this class are anatomically characterized by having **four pairs of legs** (8 legs total) in their adult stage and a body divided into two segments: a cephalothorax and an abdomen. **Analysis of Options:** * **Arachnida (Correct):** This class includes medically important vectors such as **Ticks** (Hard and Soft) and **Mites** (e.g., *Sarcoptes scabiei*, Trombiculid mites). These are wingless and lack antennae. * **Insecta (Incorrect):** This is the largest class and includes mosquitoes, flies, lice, and fleas. Insects are characterized by **three pairs of legs** (6 legs total), a body divided into three segments (head, thorax, abdomen), and usually one or two pairs of wings. * **Crustacea (Incorrect):** This class includes aquatic organisms like cyclops (intermediate host for Guinea worm). They typically possess **five or more pairs of legs** and two pairs of antennae. **High-Yield Clinical Pearls for NEET-PG:** * **Ticks vs. Insects:** Ticks (Arachnids) do not have antennae or wings, whereas insects do. * **Larval Exception:** Note that while adult Arachnids have 4 pairs of legs, the **larval stage** of ticks and mites possesses only **3 pairs**, often leading to confusion in exams. * **Disease Association:** * *Hard Ticks:* KFD, Tick paralysis, Indian Tick Typhus. * *Mites:* Scabies (Sarcoptes), Scrub Typhus (Trombiculid mite). * *Cyclops (Crustacea):* Dracunculiasis (Guinea worm) and Fish Tapeworm (Diphyllobothrium latum).

Q: Which of the following has the highest chance of transmission of HIV?

Transfusion of blood products. **Explanation:** The risk of HIV transmission depends on the **viral load** in the source fluid and the **route of exposure**. **1. Why Option B is Correct:** Transfusion of infected blood or blood products has the highest transmission efficiency, estimated at **>90% (approximately 92.5%)**. This is because a large volume of the virus is introduced directly into the recipient's bloodstream, bypassing all natural mucosal barriers. In the context of NEET-PG, blood transfusion is always considered the most efficient mode of transmission per single act of exposure. **2. Analysis of Incorrect Options:** * **Anal Intercourse (Option A):** While this is the most common mode of sexual transmission (specifically for the receptive partner), the risk per act is significantly lower, estimated at **1% to 1.4% (138 per 10,000 exposures)**. * **Kidney Transplant (Option C):** Organ transplantation carries a high risk, but it is statistically lower than a direct whole-blood transfusion. Furthermore, rigorous screening protocols make this a rare event compared to other modes. * **Transplacental Transmission (Option D):** Vertical transmission (Mother-to-Child) has a risk of **20–45%** in the absence of any intervention (ART). While high, it does not reach the near-certainty of a contaminated blood transfusion. **3. High-Yield Clinical Pearls for NEET-PG:** * **Order of Transmission Risk (Per act):** Blood Transfusion (>90%) > Vertical Transmission (20-45%) > Receptive Anal Intercourse (1%) > Needle Stick Injury (0.3%) > Receptive Vaginal Intercourse (0.08%). * **Most common route worldwide:** Heterosexual transmission. * **Most common route in India:** Heterosexual transmission. * **Needle Stick Injury Risk:** The risk of transmitting HIV via a needle stick is **0.3%**, whereas Hepatitis B (HBV) is much higher at **30%** and Hepatitis C (HCV) is **3%** (Rule of 3: 0.3, 3, 30).

Q: All the following are fishes used in biological control of vectors except?

Coelomomyces. ### Explanation The question tests your knowledge of **Biological Control Methods** used in Integrated Vector Management (IVM). Biological control involves using living organisms to reduce the population of disease-carrying vectors, primarily mosquitoes. **Why Coelomomyces is the correct answer:** **Coelomomyces** is not a fish; it is a genus of **entomopathogenic fungi**. It acts as an obligate parasite that infects and kills mosquito larvae. While it is a biological control agent, it belongs to the category of fungi/microbial control, not larvivorous fish. **Analysis of Incorrect Options (Larvivorous Fish):** Larvivorous fish are used to control mosquito breeding in permanent water bodies (wells, tanks, ponds) by feeding on larvae. * **Gambusia affinis (Mosquito fish):** The most widely used fish globally. A single Gambusia can consume about 100–300 larvae per day. It is hardy and can survive in varying water conditions. * **Poecilia reticulata (Guppy):** Another highly effective larvivorous fish. It is particularly useful because it can tolerate high levels of organic pollution (e.g., in drains and sewage-polluted water). * **Aplocheilus (Panchax):** A genus of small, surface-feeding fish native to India. They are efficient predators of mosquito larvae in freshwater habitats. **High-Yield NEET-PG Pearls:** * **Gambusia vs. Poecilia:** Use Gambusia for clean water and Poecilia for polluted water. * **Other Biological Agents:** *Bacillus thuringiensis israelensis* (BTI) and *Bacillus sphaericus* are bacteria used as biolarvicides. * **Larvivorous Fish Strategy:** This is a component of the **National Center for Vector Borne Diseases Control (NCVBDC)** strategy for malaria and dengue control. * **Criteria for Larvivorous Fish:** They must be small, hardy, drought-resistant, and prefer larvae over other food sources.

Q: DOTS indicates?

Short term treatment under supervision. **Explanation:** **DOTS** stands for **Directly Observed Treatment, Short-course**. It is the internationally recommended strategy for tuberculosis (TB) control, currently integrated under the National TB Elimination Programme (NTEP). 1. **Why Option A is correct:** * **Short-course:** The treatment duration for drug-sensitive TB is typically 6 months (2 months Intensive Phase + 4 months Continuation Phase), which is considered "short" compared to older regimens that lasted 18–24 months. * **Supervision:** The core pillar of DOTS is "Direct Observation." A trained health worker or a designated community member (DOT provider) watches the patient swallow their medication. This ensures **adherence**, prevents drug resistance (MDR-TB), and monitors for side effects. 2. **Why other options are incorrect:** * **Options B & D (Without supervision):** Self-administered treatment often leads to poor compliance, irregular dosing, and treatment failure, which are the primary drivers of the TB epidemic. * **Options C & D (Long term):** Modern chemotherapy uses potent bactericidal drugs (Rifampicin, Isoniazid), making long-term (multi-year) treatment unnecessary for standard cases. **High-Yield Clinical Pearls for NEET-PG:** * **Five Components of DOTS:** Political commitment, Good quality microscopy, Uninterrupted supply of drugs, Standardized recording/reporting, and Direct observation. * **NTEP Update:** India has moved from intermittent (thrice weekly) to **Daily Regimen** using Fixed-Dose Combinations (FDCs) based on weight bands. * **Nikshay:** The unified ICT platform for TB monitoring in India. * **Goal:** India aims to eliminate TB by **2025**, five years ahead of the global Sustainable Development Goal (2030).

Question 1

Which of the following statements regarding the tuberculin test is true?

Accepted Answer

More than 10 mm of induration in 72 hours indicates a positive test.

Answer

It is used for the diagnosis of tuberculosis.

Answer

It measures the incidence of tuberculosis.

Answer

It measures immunity status against tuberculosis.

Question 2

What is the incubation period of Diphtheria?

Accepted Answer

2-6 days

Answer

2-6 hours

Answer

2-6 weeks

Answer

2-6 months

Question 3

Which of the following statements regarding poliomyelitis is true?

Accepted Answer

It commonly spreads by the fecal-oral route.

Answer

Poliovirus is resistant to pasteurization.

Answer

For every clinical case, there may be 1000 subclinical cases in adults.

Answer

Most outbreaks of polio are due to type-3 poliovirus.

Question 4

Which of the following is true about the Anopheles mosquito?

Accepted Answer

Larva lacks a siphon tube

Answer

Breeds in artificially collected water

Answer

Also known as the tiger mosquito

Answer

Also known as the nuisance mosquito

Question 5

A 5-year-old child presents with rashes all over the body and was diagnosed with chickenpox. What feature helps differentiate chickenpox rash from smallpox rash?

Accepted Answer

Pleomorphic rash

Answer

Centrifugal distribution

Answer

Non-pleomorphic rash

Answer

Palms and soles are involved

Question 6

Which arthropod possesses four pairs of legs?

Accepted Answer

Arachnida

Answer

Insecta

Answer

Crustacea

Answer

None of the above

Question 7

Which of the following has the highest chance of transmission of HIV?

Accepted Answer

Transfusion of blood products

Answer

Anal intercourse

Answer

Kidney transplant

Answer

Transplacental transmission

Question 8

All the following are fishes used in biological control of vectors except?

Accepted Answer

Coelomomyces

Answer

Gambusia

Answer

Poccilla

Answer

Aplochelius

Question 9

DOTS indicates?

Accepted Answer

Short term treatment under supervision

Answer

Short term treatment without supervision

Answer

Long term treatment with supervision

Answer

Long term treatment without supervision

Question 10

All of the following are true about Dengue hemorrhagic fever EXCEPT:

Accepted Answer

Lamivudine is the drug of choice.

Answer

Malnutrition is protective.

Answer

It is transmitted by Aedes mosquitoes.

Answer

The causative agent belongs to the Flaviviridae group.

Infectious Diseases — MCQs

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