Infectious Diseases Practice Questions

Q: What is the principal vector of the dengue virus?

Aedes aegypti. **Explanation:** **Dengue virus (DENV)**, a flavivirus, is primarily transmitted to humans through the bite of infected female mosquitoes of the genus *Aedes*. **Aedes aegypti** is the principal vector because it is highly anthropophilic (prefers human blood), lives in close proximity to human habitations, and breeds in stagnant clean water (e.g., flower pots, discarded tires). While *Aedes albopictus* is a secondary vector, *A. aegypti* remains the most efficient transmitter due to its frequent feeding behavior. **Analysis of Incorrect Options:** * **Anopheles culicifacies:** This is the primary vector for **Malaria** in rural India. * **Anopheles stephensi:** This is the major vector for **Malaria** in urban areas of India. * **Culex molestus:** Species of the *Culex* genus are primarily responsible for transmitting **Japanese Encephalitis** and **Lymphatic Filariasis** (specifically *Culex quinquefasciatus*). **High-Yield Clinical Pearls for NEET-PG:** * **Feeding Pattern:** *Aedes* mosquitoes are "day biters," with peak activity during early morning and late afternoon. * **Nervous Feeding:** They are "interrupted feeders," meaning they bite multiple people to complete a single blood meal, leading to rapid outbreaks within a household. * **Transovarial Transmission:** The virus can pass from the female mosquito to her eggs, allowing the virus to persist even during dry seasons. * **Control:** The most effective prevention is source reduction (eliminating breeding sites) and using larvicides like **Abate (Temephos)**.

Q: What is the definition of sputum-positive tuberculosis?

One out of two sputum samples is positive.. **Explanation:** Under the current **National Tuberculosis Elimination Program (NTEP)** guidelines (formerly RNTCP), the definition of a microbiologically confirmed case of pulmonary tuberculosis has evolved to prioritize early detection and treatment initiation. **1. Why Option A is Correct:** According to the current diagnostic algorithm, a patient is classified as **Sputum Positive** if even **one** out of two sputum samples (typically one spot and one morning sample) is positive for Acid-Fast Bacilli (AFB) via Ziehl-Neelsen (ZN) staining or fluorescence microscopy. This "one-positive" criteria was adopted to increase the sensitivity of the screening process and ensure that infectious cases are not missed. **2. Why Other Options are Incorrect:** * **Option B:** The "two out of three" criteria is an outdated protocol. Requiring multiple positive smears significantly increased patient dropout rates during the diagnostic phase. * **Option C:** While BACTEC (a rapid liquid culture system) is a highly sensitive method for detecting *M. tuberculosis*, the standard clinical definition of "sputum-positive TB" specifically refers to smear microscopy findings rather than culture results. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnostic Shift:** The NTEP now prioritizes **NAAT (CBNAAT/TrueNat)** as the initial diagnostic test for TB, which detects both the organism and Rifampicin resistance. * **Sputum Grading:** A minimum of **5,000–10,000 bacilli/ml** of sputum is required for a smear to be positive under ZN staining. * **Screening:** For TB screening in the community, the most sensitive symptom is a cough of **≥ 2 weeks** duration. * **Microscopy:** Fluorescence microscopy is approximately 10% more sensitive than traditional ZN staining and allows for faster examination of slides.

Q: What is the recommended dose of chloroquine for children aged 4-8 years?

300 mg. **Explanation:** The treatment of Malaria in the National Vector Borne Disease Control Programme (NVBDCP) follows age-specific dosage schedules for Chloroquine (CQ). Chloroquine is administered at a total dose of **25 mg/kg** body weight divided over three days (10 mg/kg on Day 1, 10 mg/kg on Day 2, and 5 mg/kg on Day 3). For standardized field use, the dosage is categorized by age groups: * ** 15 years:** 600 mg (Day 1 & 2) and 300 mg (Day 3) **Analysis of Options:** * **Option A (150 mg):** This is the dose for children aged 1–4 years. * **Option C (450 mg):** This is the dose for children aged 8–12 years. * **Option D (600 mg):** This is the standard adult dose (for Day 1 and Day 2). **High-Yield Clinical Pearls for NEET-PG:** 1. **Standard Tablet:** One tablet of Chloroquine phosphate contains **150 mg base** (250 mg salt). 2. **Mechanism:** It inhibits heme polymerase, leading to the accumulation of toxic heme in the parasite. 3. **Contraindication:** Chloroquine is avoided in patients with psoriasis and retinal diseases. 4. **Pregnancy:** Chloroquine is safe and remains the drug of choice for *P. vivax* in pregnancy. 5. **Radical Cure:** For *P. vivax*, Chloroquine is followed by Primaquine (0.25 mg/kg) for 14 days to prevent relapse (except in G6PD deficiency and pregnancy).

Q: What is/are true regarding the management of a rat bite?

Rarely need rabies immunoglobulin. **Explanation:** The management of rat bites is a frequent point of confusion in medical exams. The core concept to understand is that **small rodents (rats, mice, squirrels, hamsters) are not known to transmit rabies to humans.** **1. Why Option C is Correct:** According to the **WHO and National Guidelines on Rabies Prophylaxis**, small rodents are generally considered "low-risk" or "no-risk" for rabies. While they can theoretically be infected, they usually die from the predator's bite before they can transmit the virus. Therefore, **Post-Exposure Prophylaxis (PEP), including Rabies Immunoglobulin (RIG) and the Rabies Vaccine, is not routinely recommended** for rat bites. RIG is "rarely" needed unless the animal was confirmed to be rabid or showed highly unusual behavior in an endemic area. **2. Why Other Options are Incorrect:** * **Option A:** Routine vaccination with ARV is not indicated for rat bites for the reasons mentioned above. * **Option B:** While Tetanus Toxoid (TT) is part of general wound management, it is not the *defining* or most specific management priority unique to the clinical dilemma of "rabies vs. no rabies" in rat bites. * **Option D:** Washing with soap and water is standard for any wound, but in the context of this specific question, the examiner is testing your knowledge of rabies protocol. **High-Yield Clinical Pearls for NEET-PG:** * **Rat-Bite Fever:** The primary concern in rat bites is not rabies, but infections caused by *Streptobacillus moniliformis* or *Spirillum minus*. * **Rabies-Free Animals:** Bites from rabbits, hares, and small rodents do **not** require rabies PEP. * **Treatment of Choice:** For Rat-Bite Fever, the drug of choice is **Penicillin G**. * **Category III Bites:** For high-risk animals (dogs, cats, monkeys), RIG is mandatory for all Category III exposures. For rats, it is omitted.

Q: Which of the following diseases is transmitted by *Phlebotomus argentipes*?

Kala-azar. **Explanation:** The correct answer is **Kala-azar** (Visceral Leishmaniasis). *Phlebotomus argentipes* is the primary insect vector for Kala-azar in the Indian subcontinent. It is a species of **sandfly** that breeds in damp soil, cracks in mud walls, and cattle sheds. The sandfly transmits the protozoan parasite *Leishmania donovani* to humans through its bite. In India, the disease is endemic in states like Bihar, Jharkhand, West Bengal, and Uttar Pradesh. **Analysis of Incorrect Options:** * **A. Epidemic typhus:** Caused by *Rickettsia prowazekii* and transmitted by the **Body louse** (*Pediculus humanus corporis*). * **C. Relapsing fever:** Louse-borne relapsing fever is caused by *Borrelia recurrentis* (transmitted by the **Body louse**), while tick-borne relapsing fever is transmitted by **Ornithodoros ticks**. * **D. Trench fever:** Caused by *Bartonella quintana* and transmitted by the **Body louse**. **High-Yield Clinical Pearls for NEET-PG:** * **Vector Characteristics:** Sandflies are small (1/4th the size of a mosquito), hop rather than fly, and are nocturnal. Only the **female** sandfly bites. * **Control Measures:** The most effective method to control *Phlebotomus* is **Indoor Residual Spraying (IRS)** with DDT (though resistance is emerging) or synthetic pyrethroids. * **Other Sandfly Diseases:** Besides Kala-azar, sandflies also transmit Oriental Sore (Cutaneous Leishmaniasis) and Sandfly fever (Pappataci fever). * **Diagnostic Tip:** In Kala-azar, look for the "Pentad": Fever, hepatosplenomegaly, pancytopenia, hypergammaglobulinemia, and darkening of the skin.

Q: Which of the following rodents is the natural reservoir of plague?

Tatera indica. ### Explanation **Correct Answer: B. Tatera indica** In the epidemiology of plague (*Yersinia pestis*), it is crucial to distinguish between the **natural (sylvatic) reservoir** and the **commensal (urban) hosts**. 1. **Why Tatera indica is correct:** * *Tatera indica* (the Indian Gerbil) is the **natural reservoir** of plague in India. * Natural reservoirs are wild rodents that are relatively resistant to the disease. They maintain the infection in a "silent" state in nature (enzootic foci) for long periods without dying out, ensuring the long-term survival of the bacteria. 2. **Why the other options are incorrect:** * **C. Rattus rattus (Roof Rat):** This is a commensal peridomestic rodent. It is highly susceptible to plague and usually dies during an outbreak. It acts as the **bridge** that brings the infection from the wild to human dwellings. * **D. Rattus norvegicus (Norway Rat/Sewer Rat):** Similar to *R. rattus*, it is a domestic rodent that acts as a host during urban outbreaks but is not the primary natural reservoir. * **A. Mus musculus (House Mouse):** While it can be infected, it plays a negligible role in the maintenance or transmission of plague compared to *Tatera* or *Rattus* species. ### High-Yield Clinical Pearls for NEET-PG: * **The "Rat Fall":** A sudden increase in the death of commensal rats (*R. rattus*) serves as a warning sign of an impending human plague epidemic. * **Vector:** The most efficient vector for transmitting plague from rat to man is the **Xenopsylla cheopis** (Oriental rat flea). * **Flea Index:** An "Anterior Flea Index" (specifically the *X. cheopis* index) of **>1** is considered a critical threshold for an increased risk of a plague outbreak. * **Drug of Choice:** Streptomycin is the traditional drug of choice; however, Gentamicin or Doxycycline are also frequently used.

Q: Which of the following is NOT true about dengue fever?

Its occurrence is unaffected by ambient temperature.. ### Explanation **Correct Answer: C. Its occurrence is unaffected by ambient temperature.** **1. Why Option C is the correct (False) statement:** The transmission of Dengue is **highly sensitive** to ambient temperature. The life cycle of the vector (*Aedes aegypti*) and the **Extrinsic Incubation Period (EIP)** of the virus are temperature-dependent. Higher temperatures (ideally between 20°C and 30°C) shorten the EIP, allowing the mosquito to become infectious faster. Conversely, temperatures below 16°C or above 40°C significantly inhibit mosquito activity and viral replication. Therefore, saying it is "unaffected" is epidemiologically incorrect. **2. Analysis of Incorrect Options:** * **Option A:** Dengue is indeed the **most common arboviral disease** in the world, with an estimated 3.9 billion people at risk across over 120 countries. * **Option B:** It exhibits both patterns. It is **endemic** in tropical regions (like India) due to year-round vector presence and becomes **epidemic** during monsoon and post-monsoon seasons when stagnant water increases breeding sites. * **Option C:** While complications like Dengue Hemorrhagic Fever (DHF) can be fatal, Classical Dengue Fever is typically a **self-limiting** febrile illness lasting 2–7 days. **3. High-Yield Clinical Pearls for NEET-PG:** * **Vector:** *Aedes aegypti* (Tiger mosquito) – a day biter, breeds in artificial collections of clean water. * **Virus:** Flavivirus (4 serotypes: DEN 1, 2, 3, 4). Type 2 is most commonly associated with DHF. * **Incubation Period:** 3 to 14 days (Intrinsic). * **Diagnosis:** NS1 Antigen (Day 1-5); IgM/IgG ELISA (after Day 5). * **Saddle-back fever:** A characteristic biphasic fever pattern seen in some patients. * **Tourniquet Test:** A positive test (≥10 petechiae/square inch) indicates capillary fragility, a hallmark of DHF.

Q: Which of the following has the least chance of transmitting HIV infection?

Heterosexual intercourse. ### Explanation The risk of HIV transmission varies significantly depending on the route of exposure and the viral load of the source. This question asks for the **least** efficient mode of transmission among the given options. **1. Why Heterosexual Intercourse is the Correct Answer:** While heterosexual intercourse is the most common mode of transmission globally due to the sheer frequency of the act, it has the **lowest per-act risk** (approximately 0.04% to 0.1% for females and 0.04% for males). The low efficiency is due to the mucosal barrier and the relatively low concentration of the virus in vaginal or seminal fluids compared to whole blood. **2. Analysis of Incorrect Options:** * **Blood Transfusion:** This is the **most efficient** mode of transmission. A single unit of HIV-infected blood carries a transmission risk of approximately **90-95%**. * **Vertical Transmission:** Without intervention, the risk of mother-to-child transmission (MTCT) is about **20-45%**. With modern ART and PMTCT protocols, this can be reduced to Vertical (30%) > Needle prick (0.3%) > Receptive Anal Sex (1.38%) > Receptive Vaginal Sex (0.08%). * **Most Common Route:** Globally and in India, **Heterosexual transmission** is the most common route. * **Window Period:** Usually 2–12 weeks; the best screening test is **4th Gen ELISA** (detects p24 antigen + antibodies). * **Post-Exposure Prophylaxis (PEP):** Must be started within **72 hours** (ideally within 2 hours) and continued for **28 days**.

Q: Which of the following statements regarding the composition of new WHO oral rehydration solution (ORS) is incorrect?

Total osmolarity: 300 mmol/L. **Explanation:** The question asks for the **incorrect** statement regarding the composition of the **New (Reduced Osmolarity) WHO ORS**. **1. Why Option D is the Correct Answer (The Incorrect Statement):** The total osmolarity of the New WHO ORS is **245 mmol/L**, not 300 mmol/L. The WHO shifted from the standard ORS (311 mmol/L) to a reduced osmolarity formula to prevent hypertonicity. This change reduces stool output, decreases vomiting, and minimizes the need for unscheduled IV fluids, making it safer and more effective for children with non-cholera diarrhea. **2. Why Other Options are Incorrect (They are Correct Compositions):** * **Option A (Sodium Chloride - 2.6 g/L):** This is the correct weight for sodium chloride in the new formula. It provides 75 mmol/L of Sodium. * **Option B (Potassium Chloride - 1.5 g/L):** This is the correct weight, providing 20 mmol/L of Potassium to replace losses and prevent hypokalemia. * **Option C (Glucose - 13.5 g/L):** This is the correct weight for anhydrous glucose. It provides 75 mmol/L of glucose, maintaining a 1:1 molar ratio with sodium to optimize co-transport in the small intestine. **High-Yield Clinical Pearls for NEET-PG:** * **Composition in mmol/L:** Sodium (75), Chloride (65), Glucose (75), Potassium (20), Citrate (10). **Total = 245 mmol/L.** * **Trisodium Citrate (2.9 g/L):** Added to the ORS to correct acidosis and increase the shelf life of the packet. * **Rehydration Phase:** In Plan B (Some Dehydration), the amount of ORS given in the first 4 hours is **Weight (kg) × 75 ml**. * **Zinc Supplementation:** Always given alongside ORS (20 mg/day for 10–14 days; 10 mg for infants <6 months) to reduce the duration and recurrence of diarrhea.

Question 1

What is the principal vector of the dengue virus?

Accepted Answer

Aedes aegypti

Answer

Anopheles culicifacies

Answer

Anopheles stephensi

Answer

Culex molestus

Question 2

What is the definition of sputum-positive tuberculosis?

Accepted Answer

One out of two sputum samples is positive.

Answer

Two out of three sputum samples are positive.

Answer

BACTEC positive.

Answer

None of the above.

Question 3

What is the recommended dose of chloroquine for children aged 4-8 years?

Accepted Answer

300 mg

Answer

150 mg

Answer

450 mg

Answer

600 mg

Question 4

What is/are true regarding the management of a rat bite?

Accepted Answer

Rarely need rabies immunoglobulin

Answer

Vaccinate with antirabies vaccine

Answer

Give tetanus toxoid

Answer

Wash with soap and use antiseptic cream

Question 5

Which of the following diseases is transmitted by *Phlebotomus argentipes*?

Accepted Answer

Kala-azar

Answer

Epidemic typhus

Answer

Relapsing fever

Answer

Trench fever

Question 6

Which of the following rodents is the natural reservoir of plague?

Accepted Answer

Tatera indica

Answer

Mus musculus

Answer

Rattus Rattus

Answer

Rattus norvegicus

Question 7

Which of the following is NOT true about dengue fever?

Accepted Answer

Its occurrence is unaffected by ambient temperature.

Answer

It is the most common arboviral disease globally.

Answer

It exhibits both endemic and epidemic patterns.

Answer

It is typically a self-limiting illness.

Question 8

Which of the following has the least chance of transmitting HIV infection?

Accepted Answer

Heterosexual intercourse

Answer

Blood transfusion

Answer

Vertical transmission

Answer

IV drug abuse

Question 9

Which of the following statements regarding the composition of new WHO oral rehydration solution (ORS) is incorrect?

Accepted Answer

Total osmolarity: 300 mmol/L

Answer

Sodium chloride: 2.6 grams/litre

Answer

Potassium chloride: 1.5 grams/litre

Answer

Glucose: 13.5 grams/litre

Question 10

Which of the following is resistant to DDT, except?

Accepted Answer

Phlebotomus

Answer

Culex fatigans

Answer

Anopheles stephensi

Answer

Musca domestica

Infectious Diseases — MCQs

Infectious Diseases — MCQs

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