Infectious Diseases Practice Questions

Q: Amphixenosis is best described as:

An infection caused by Trypanosoma cruzi. **Explanation:** **Understanding the Concept:** Zoonoses are classified based on the direction of transmission between hosts. **Amphixenosis** refers to an infection that is maintained in nature by both humans and lower vertebrate animals, where the transmission can occur in either direction (Humans ↔ Animals). **Why Option D is Correct:** * **Trypanosoma cruzi (Chagas Disease):** This is a classic example of amphixenosis. It circulates among a wide range of wild and domestic animals (reservoirs) and humans, with the Triatomine bug acting as the vector. The infection cycle is equally sustainable in both human and animal populations. **Analysis of Incorrect Options:** * **Option A & C (Enterobius vermicularis & Ascaris lumbricoides):** These are **Anthroponoses**. These infections are restricted to humans; they do not have an animal reservoir and are transmitted from human to human. * **Option B (Bacillus anthracis):** This is an **Anthropozoonosis**. This term refers to infections that are primarily maintained in animals and are transmitted from animals to humans (Animals → Humans). Humans are usually "dead-end" hosts. **High-Yield Clinical Pearls for NEET-PG:** * **Zooanthroponosis:** Infections transmitted from humans to animals (e.g., Human Tuberculosis to cattle). * **Cyclozoonosis:** Requires more than one vertebrate host species to complete the life cycle (e.g., *Taenia saginata*, *Echinococcus granulosus*). * **Metazoonosis:** Transmitted to humans via an invertebrate vector (e.g., Plague, Arboviruses). * **Saprozoonosis:** Requires a non-animal environmental reservoir like soil or water (e.g., Histoplasmosis, Larva migrans).

Q: A housefly lands on cow dung, contaminating its appendages with bacteria from the faeces, and then lands on food prior to consumption. This is an example of which type of disease transmission?

Mechanical. ### Explanation **Correct Answer: C. Mechanical** **1. Why Mechanical Transmission is Correct:** Mechanical transmission occurs when a pathogen is physically carried from one location to another by a vector (like a housefly or cockroach) without any biological development or multiplication of the pathogen within the vector. In this scenario, the housefly acts as a "passive carrier," transporting bacteria from cow dung to food via its legs, wings, or mouthparts. The pathogen undergoes no change in its life cycle during this transit. **2. Why Other Options are Incorrect:** These options represent **Biological Transmission**, where the pathogen must undergo a specific biological process within the vector: * **Propagative (D):** The pathogen multiplies in number but does not change its stage or form (e.g., Plague bacilli in rat fleas). * **Cyclo-developmental (A):** The pathogen undergoes developmental changes (morphological transformation) but does not multiply (e.g., Filarial worms in *Culex* mosquitoes). * **Cyclo-propagative (B):** The pathogen undergoes both developmental changes and multiplication (e.g., *Plasmodium* in *Anopheles* mosquitoes). **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Common Mechanical Vectors:** *Musca domestica* (Housefly) and *Blattaria* (Cockroaches). * **Diseases transmitted mechanically:** Typhoid (Salmonella), Cholera, Amoebiasis, and Trachoma. * **Key Distinction:** If the vector is essential for the pathogen's life cycle, it is **Biological**. If the vector is merely a transport vehicle, it is **Mechanical**. * **Extrinsic Incubation Period:** This term applies only to biological transmission; it is the time required for the pathogen to develop/multiply within the vector before it becomes infective.

Q: All are true regarding typhoid fever, except?

Peak incidence in 30-50 years. **Explanation:** Typhoid fever, caused by *Salmonella Typhi*, is a systemic infection primarily transmitted via the feco-oral route. **1. Why Option A is the Correct Answer (The False Statement):** The peak incidence of typhoid fever is **not** in the 30–50 age group. It is primarily a disease of **children and young adults**, with the highest incidence typically seen in the **5–19 years** age group. In endemic areas, children under 5 also carry a significant burden. By age 30, many individuals have developed partial immunity due to repeated subclinical exposures. **2. Analysis of Other Options:** * **Option B (No non-human reservoir):** This is **true**. *Salmonella Typhi* and *Paratyphi* are strictly human pathogens. There are no animal or environmental reservoirs; transmission occurs only from human cases or carriers. * **Option C (Carriers can exist for years):** This is **true**. Approximately 1–5% of patients become chronic carriers (defined as excreting bacilli in stools for >1 year). The **gallbladder** is the most common site of colonization, often associated with gallstones. * **Option D (Lifelong immunity cannot be acquired):** This is **true**. Natural infection does not confer permanent immunity. Re-infection can occur if the inoculum size is large, which is why vaccination and sanitation remain critical even in previously infected individuals. **NEET-PG High-Yield Pearls:** * **Investigation of Choice:** Blood culture is the gold standard in the 1st week; Stool culture in the 3rd week. * **Pathognomonic sign:** Rose spots (seen in the 2nd week). * **Drug of Choice:** Ceftriaxone (due to widespread multidrug resistance to Ciprofloxacin). * **Carrier State:** More common in females and those with biliary tract abnormalities. Treatment of choice for carriers is Ciprofloxacin for 6 weeks or Cholecystectomy.

Q: Which of the following is NOT an epidemiological feature of Japanese encephalitis?

Infected pigs manifest the disease.. **Explanation:** Japanese Encephalitis (JE) is a zoonotic viral infection caused by a Group B Arbovirus (Flavivirus). Understanding its transmission cycle is crucial for NEET-PG. **Why Option C is the correct answer (The False Statement):** In the transmission cycle of JE, **pigs act as "Amplifier Hosts."** While the virus multiplies rapidly in their blood (viremia), it does not cause clinical disease in them. They remain **asymptomatic**, making them dangerous reservoirs that facilitate the infection of mosquito vectors (*Culex tritaeniorhynchus*). **Analysis of Incorrect Options:** * **Option A (Extra-human hosts):** This is true. The natural cycle involves birds (Ardeid birds like herons and egrets) and animals (pigs). * **Option B (Man is an incidental host):** This is true. Humans are "dead-end hosts" because the level of viremia in humans is insufficient to infect a biting mosquito. Humans do not play a role in maintaining the transmission cycle. * **Option D (Epidemics in Karnataka):** This is true. JE is endemic in several Indian states, including Uttar Pradesh, Bihar, West Bengal, and Southern states like Karnataka, Andhra Pradesh, and Tamil Nadu. **High-Yield Clinical Pearls for NEET-PG:** * **Vector:** *Culex tritaeniorhynchus* (breeds in stagnant water/paddy fields). * **Biting Habit:** Exophagic (outdoors) and Zoophilic (prefers animals). * **Amplifier Host:** Pig (Most important for human outbreaks). * **Reservoir/Maintenance Host:** Ardeid birds. * **Vaccination:** Under the Universal Immunization Programme (UIP), the **SA-14-14-2** (Live attenuated) vaccine is used in India. * **Seasonality:** Often coincides with the monsoon and post-monsoon periods due to increased mosquito breeding in rice fields.

Q: What is the average number of mites found on the body of a person suffering from scabies?

10-15. **Explanation:** **1. Why Option C (10-15) is Correct:** Scabies is a parasitic infestation caused by the mite *Sarcoptes scabiei* var. *hominis*. In a typical case of "classical" or "ordinary" scabies in an immunocompetent adult, the total number of adult female mites burrowing in the skin is surprisingly low, averaging between **10 to 15 mites**. Despite this small number, the intense pruritus (itching) experienced by the patient is disproportionately severe because it is a type IV hypersensitivity reaction to the mites, their eggs, and their scybala (feces). **2. Analysis of Incorrect Options:** * **Options A (2-4) and B (5-9):** These numbers are too low. While the infestation starts with a single impregnated female, by the time clinical symptoms manifest (usually 3–6 weeks after primary exposure), the population typically stabilizes around 10–15. * **Option D (15-20):** While some patients may harbor slightly more mites, the standard epidemiological and dermatological consensus cited in textbooks (like Park’s PSM) specifically identifies the 10–15 range as the average. **3. Clinical Pearls for NEET-PG:** * **Norwegian (Crusted) Scabies:** This is a high-yield exception. In immunocompromised individuals (e.g., HIV, transplant recipients), the immune system fails to check the mite replication, leading to **millions of mites** on the body. It is highly contagious. * **Incubation Period:** 3–6 weeks for primary infestation; 1–4 days for re-infestation. * **Pathognomonic Sign:** The **Burrow** (a short, wavy, dirty-looking line). * **Treatment of Choice:** Permethrin 5% cream (applied neck down, left for 8–12 hours). * **Public Health Note:** Always treat all close contacts simultaneously to prevent "ping-pong" reinfection, even if they are asymptomatic.

Q: After initiation of DOTS therapy for category-I, when is the first sputum examination done?

2 months of treatment. **Explanation:** Under the National Tuberculosis Elimination Program (NTEP), the monitoring of treatment response for **Category-I (New cases)** is primarily done through follow-up sputum smear microscopy. **Why Option B is correct:** Category-I treatment consists of an **Intensive Phase (IP)** of 2 months (HRZE) followed by a **Continuation Phase (CP)** of 4 months (HRE). The first follow-up sputum examination is scheduled at the **end of the Intensive Phase (2 months)**. This is a critical milestone to assess "sputum conversion" (from positive to negative), which indicates the effectiveness of the treatment and a reduction in the patient's infectivity. **Why other options are incorrect:** * **Option A (1 month):** This is too early to assess microbiological conversion for standard pulmonary TB. * **Option C (4 months):** Under previous guidelines, a smear was done at 4 months, but current protocols emphasize testing at the end of IP (2 months) and at the end of treatment (6 months). * **Option D (6 months):** This is the timing for the **final** sputum examination to declare the patient "Cured." **High-Yield Clinical Pearls for NEET-PG:** * **Follow-up Schedule:** For New cases, sputum is checked at **2 months** (end of IP) and **6 months** (end of CP). * **If Smear is Positive at 2 Months:** In the current NTEP algorithm, if the 2-month smear is positive, the patient is still transitioned to the Continuation Phase, but a **NAAT (CBNAAT/Truenat)** must be performed to rule out Drug-Resistant TB (DR-TB). * **Definition of "Cured":** A patient who was initially smear-positive, completed treatment, and had a negative smear at the end of treatment (6 months) plus at least one previous negative result.

Q: Which of the following are not sexually transmitted infections?

Hepatitis A virus (HAV) and Varicella zoster virus (VZV). **Explanation:** The core concept of this question lies in distinguishing between pathogens primarily transmitted through sexual contact and those transmitted via respiratory or feco-oral routes. **Why Option A is Correct:** * **Hepatitis A Virus (HAV):** It is primarily transmitted via the **feco-oral route** (contaminated food/water). While sexual transmission can occur among men who have sex with men (MSM) due to oral-anal contact, it is not classified as a classic Sexually Transmitted Infection (STI). * **Varicella Zoster Virus (VZV):** This virus causes Chickenpox and Herpes Zoster. It is transmitted via **respiratory droplets** or direct contact with vesicle fluid. It is never classified as an STI. **Analysis of Incorrect Options:** * **Option B (HIV):** HIV is a classic STI, transmitted through blood, semen, and vaginal fluids. * **Option C (HTLV-I):** This retrovirus is transmitted through sexual contact, blood transfusion, and breastfeeding. It is a recognized STI. * **Option D (HPV):** Human Papillomavirus is one of the most common STIs globally, leading to genital warts and cervical cancer. **NEET-PG High-Yield Pearls:** * **Hepatitis & Sex:** Hepatitis B, C, and D are considered STIs (HBV being the most sexually transmissible). Hepatitis A and E are feco-oral. * **Ulcerative vs. Non-Ulcerative STIs:** Remember that Syphilis, Chancroid, and LGV are ulcerative, while Gonorrhea and Chlamydia are typically non-ulcerative. * **VZV Transmission:** VZV is highly contagious via the respiratory route *before* the rash appears. Once lesions crust over, they are no longer infectious.

Q: Rabies can be transmitted by all EXCEPT:

Bite of squirrel. **Explanation:** Rabies is a fatal viral zoonosis caused by the **Lyssavirus (Rhabdoviridae family)**. The virus is primarily maintained in nature by mammalian reservoirs, specifically those in the orders **Carnivora** (dogs, foxes, raccoons) and **Chiroptera** (bats). **Why Option D is correct:** Small rodents such as **squirrels**, hamsters, guinea pigs, gerbils, rats, and mice, as well as lagomorphs (rabbits and hares), are almost never found to be infected with rabies. Furthermore, they have not been known to transmit rabies to humans. According to the CDC and WHO, bites from these animals are generally not considered an indication for Rabies Post-Exposure Prophylaxis (PEP). **Why other options are incorrect:** * **Option A (Dog):** Domestic dogs are the most common source of rabies transmission to humans worldwide (responsible for >99% of human cases). * **Option B (Bat):** Bats are a significant reservoir, especially in the Americas. They can transmit the virus through bites that are often so small they go unnoticed. * **Option C (Fox):** Foxes, along with wolves and jackals, are major sylvatic (wildlife) reservoirs of the virus. **High-Yield Clinical Pearls for NEET-PG:** 1. **Negri Bodies:** Pathognomonic intracytoplasmic inclusion bodies found in the hippocampus (Ammon’s horn) and cerebellum (Purkinje cells). 2. **Street Virus vs. Fixed Virus:** Street virus is the natural isolate with long incubation; Fixed virus is laboratory-attenuated (Pasteur) used for vaccine production. 3. **Hydrophobia:** Pathognomonic clinical sign caused by forceful spasms of the diaphragm and accessory respiratory muscles when attempting to swallow. 4. **Post-Exposure Prophylaxis (PEP):** Includes wound washing (most important), Rabies Immunoglobulin (RIG), and modern cell culture vaccines (Day 0, 3, 7, 14, 28).

Q: Which of the following exposures carries the maximum risk of transmission of HIV?

Blood transfusion. The risk of HIV transmission depends on the route of exposure, the viral load in the source fluid, and the volume of inoculum. **Correct Option: A. Blood Transfusion** Blood transfusion carries the highest risk of HIV transmission, estimated at approximately **90–92% per exposure**. This is because a large volume of infected blood is introduced directly into the recipient's systemic circulation. When a unit of blood contaminated with HIV is transfused, seroconversion is almost certain. **Explanation of Incorrect Options:** * **B. Sexual Intercourse:** While this is the most common mode of transmission globally, the risk per single act is relatively low. Receptive anal intercourse carries the highest risk among sexual acts (~1.38%), followed by receptive vaginal intercourse (~0.08%). * **C. Needle Prick:** The average risk of HIV transmission after a percutaneous exposure to HIV-infected blood (e.g., accidental needlestick) is approximately **0.3%**. For mucous membrane exposure, it is even lower (~0.09%). * **D. Transplacental:** Mother-to-child transmission (MTCT) can occur during pregnancy, delivery, or breastfeeding. Without intervention, the risk ranges from **15% to 45%**; however, with effective Antiretroviral Therapy (ART), this can be reduced to Vertical Transmission (without ART) > Receptive Anal Sex > Needle Prick > Receptive Vaginal Sex. * **Post-Exposure Prophylaxis (PEP):** Should be started as soon as possible, ideally within **2 hours** and no later than **72 hours**, continuing for **28 days**. * **Standard PEP Regimen:** Tenofovir (300mg) + Lamivudine (300mg) + Dolutegravir (50mg) once daily.

Question 1

Amphixenosis is best described as:

Accepted Answer

An infection caused by Trypanosoma cruzi

Answer

An infection caused by Enterobius vermicularis

Answer

An infection caused by Bacillus anthracis (Anthrax)

Answer

An infection caused by Ascaris lumbricoides

Question 2

A housefly lands on cow dung, contaminating its appendages with bacteria from the faeces, and then lands on food prior to consumption. This is an example of which type of disease transmission?

Accepted Answer

Mechanical

Answer

Cycl developmental

Answer

Cyclo propagative

Answer

Propagative

Question 3

All are true regarding typhoid fever, except?

Accepted Answer

Peak incidence in 30-50 years

Answer

No non-human reservoir

Answer

Carriers can exist for years

Answer

Lifelong immunity cannot be acquired

Question 4

Which of the following is NOT an epidemiological feature of Japanese encephalitis?

Accepted Answer

Infected pigs manifest the disease.

Answer

The virus infects extra-human hosts.

Answer

Man is an incidental host.

Answer

Epidemics have been reported in Karnataka.

Question 5

What is the average number of mites found on the body of a person suffering from scabies?

Accepted Answer

10-15

Answer

2-4

Answer

5-9

Answer

15-20

Question 6

After initiation of DOTS therapy for category-I, when is the first sputum examination done?

Accepted Answer

2 months of treatment

Answer

1 month of treatment

Answer

4 months of treatment

Answer

6 months of treatment

Question 7

Which of the following are not sexually transmitted infections?

Accepted Answer

Hepatitis A virus (HAV) and Varicella zoster virus (VZV)

Answer

Human Immunodeficiency Virus (HIV)

Answer

Hepatitis A virus (HAV) and Human T-lymphotropic virus type I (HTLV-I)

Answer

Hepatitis A virus (HAV) and Human Papillomavirus (HPV)

Question 8

Rabies can be transmitted by all EXCEPT:

Accepted Answer

Bite of squirrel

Answer

Bite of rabid dog

Answer

Bite of bats

Answer

Bite of fox

Question 9

Which diseases are transmitted by the Culex mosquito?

Accepted Answer

Dengue

Answer

Malaria

Answer

Filariasis

Answer

Japanese encephalitis

Question 10

Which of the following exposures carries the maximum risk of transmission of HIV?

Accepted Answer

Blood transfusion

Answer

Sexual intercourse

Answer

Needle prick

Answer

Transplacental

Infectious Diseases — MCQs

Infectious Diseases — MCQs

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