Infectious Diseases Practice Questions

Q: Which of the following is NOT spread by Aedes mosquito?

Japanese encephalitis. **Explanation:** The correct answer is **Japanese encephalitis (JE)** because it is transmitted by the bite of infected **Culex mosquitoes**, primarily *Culex tritaeniorhynchus* and *Culex vishnui*. These mosquitoes typically breed in stagnant water, such as rice fields and shallow ditches. **Why the other options are incorrect:** * **Dengue fever:** This is the most common viral disease transmitted by the **Aedes aegypti** (primary vector) and *Aedes albopictus* mosquitoes. * **Chikungunya:** This viral illness is also transmitted to humans by the bite of infected **Aedes aegypti** and **Aedes albopictus** mosquitoes. * **Yellow fever:** This is a viral hemorrhagic disease transmitted by **Aedes aegypti** in urban cycles and *Haemagogus* species in jungle cycles. **High-Yield Clinical Pearls for NEET-PG:** * **Aedes Characteristics:** Known as the "Tiger Mosquito" due to white stripes on its body. It is a **day-biter** (mostly early morning and late afternoon) and breeds in **artificial collections of clean water** (e.g., flower pots, discarded tires, desert coolers). * **Culex Characteristics:** Known as the "nuisance mosquito." It is a **night-biter** and breeds in **dirty/polluted water**. Besides JE, it is the vector for **Bancroftian Filariasis** and West Nile Virus. * **Anopheles:** The vector for **Malaria**; it breeds in fresh water and bites at night. * **Zika Virus:** Also transmitted by the *Aedes* mosquito.

Q: Which of the following is NOT a characteristic of Reduced Osmolarity Oral Rehydration Solution (ORS)?

Chloride 45 mmol/litre. **Explanation** The WHO and UNICEF recommended the **Reduced Osmolarity ORS** in 2002 to replace the standard ORS. This change was aimed at reducing the need for intravenous fluids, decreasing stool output, and minimizing the incidence of vomiting in children with non-cholera diarrhea. **Why Option C is the Correct Answer:** The Chloride content in Reduced Osmolarity ORS is **65 mmol/L**, not 45 mmol/L. Therefore, Option C is the incorrect statement regarding the composition. The reduction in chloride (from 80 mmol/L in the old formula to 65 mmol/L) helps in achieving the lower total osmolarity. **Analysis of Incorrect Options:** * **Option A (Sodium 75 mmol/L):** This is a key feature. The sodium concentration was reduced from 90 mmol/L to 75 mmol/L to prevent hypernatremia. * **Option B (Potassium 20 mmol/L):** This remains unchanged from the original formula. Potassium is essential to replace losses and prevent hypokalemia during diarrhea. * **Option D (Osmolarity 245 mOsm/L):** This is the total osmolarity of the new formula (reduced from 311 mOsm/L). Lower osmolarity enhances water absorption and prevents osmotic aggravation of diarrhea. **High-Yield NEET-PG Pearls:** * **Composition of Reduced Osmolarity ORS (per litre):** * Sodium Chloride: 2.6 g * Glucose (Anhydrous): 13.5 g * Potassium Chloride: 1.5 g * Trisodium Citrate: 2.9 g * **Molar Concentrations:** Sodium (75), Chloride (65), Glucose (75), Potassium (20), Citrate (10). **Total Osmolarity = 245 mmol/L.** * **Glucose:Sodium Ratio:** Always **1:1** in the new formula to optimize the SGLT-1 co-transport mechanism in the small intestine. * **Trisodium Citrate:** Preferred over Bicarbonate because it increases the shelf life of ORS packets.

Q: Which is the main vector of malaria in urban areas?

Anopheles stephensi. **Explanation:** The correct answer is **Anopheles stephensi**. In India, the distribution of malaria vectors is highly habitat-specific, which is a frequent high-yield topic in NEET-PG. **Why Anopheles stephensi is correct:** *Anopheles stephensi* is the primary vector for **urban malaria**. It has adapted to thrive in artificial containers, overhead water tanks, cisterns, and construction sites common in densely populated cities. It is highly resilient and can breed in even small amounts of clean, stagnant water found in urban environments. **Analysis of Incorrect Options:** * **Anopheles culicifacies:** This is the most important vector for **rural malaria** in India. It accounts for nearly 60-70% of total malaria cases in the country, breeding primarily in rainwater pools, irrigation channels, and borrow pits. * **Anopheles fluviatilis:** This is the major vector in **hilly areas, forests, and foot-hills**. It prefers breeding in slow-moving streams and is known for its high anthropophilic (human-biting) index. * **Anopheles epiroticus (formerly part of the A. sundaicus complex):** This vector is responsible for malaria in **coastal areas**, as it breeds in brackish (salty) water, particularly in the Andaman and Nicobar Islands. **High-Yield Clinical Pearls for NEET-PG:** * **Rural Malaria:** *A. culicifacies* * **Urban Malaria:** *A. stephensi* * **Hilly/Forest Malaria:** *A. fluviatilis* and *A. minimus* * **Coastal Malaria:** *A. sundaicus / A. epiroticus* * **Major Vector in North-East India:** *Anopheles minimus* and *Anopheles dirus* (the latter is a forest-dweller and an efficient "exophilic" vector). * **Time of Biting:** Most Anopheles mosquitoes are nocturnal (bite between dusk and dawn).

Q: What is the main period of communicability of whooping cough?

Catarrhal stage. **Explanation:** **Whooping Cough (Pertussis)**, caused by *Bordetella pertussis*, is highly contagious. The period of communicability extends from the beginning of the catarrhal stage to approximately 3 weeks after the onset of the paroxysmal stage. **Why the Catarrhal Stage is Correct:** The **Catarrhal stage** (lasting 1–2 weeks) is characterized by non-specific symptoms like coryza, sneezing, and a mild cough. During this phase, the bacterial load in the nasopharynx is at its **peak**. Because the symptoms mimic a common cold, patients remain active and undiagnosed, leading to maximum transmission through respiratory droplets. **Analysis of Incorrect Options:** * **Incubation Period:** This is the time between infection and the onset of symptoms (usually 7–14 days). The bacteria are multiplying, but the patient is not yet shedding enough pathogens to be considered the "main" period of communicability. * **Paroxysmal Stage:** While the patient is still infectious during the first 2–3 weeks of this stage, the bacterial count begins to decline significantly as the body’s immune response kicks in. The characteristic "whoop" appears here, but the peak infectiousness has already passed. * **Convalescent Stage:** This is the recovery phase. Unless a secondary infection occurs, the patient is generally no longer infectious by this stage (usually 4 weeks after onset). **NEET-PG High-Yield Pearls:** * **Drug of Choice:** Erythromycin (or other Macrolides like Azithromycin) for 7–14 days. If started in the catarrhal stage, it can abort the disease; if started in the paroxysmal stage, it only reduces communicability. * **Secondary Attack Rate (SAR):** Very high, approximately **80–90%** among susceptible household contacts. * **Vaccine:** Part of the Pentavalent/DPT vaccine. The **acellular (aP)** component is preferred in older children/adults due to fewer side effects compared to the whole-cell (wP) version.

Q: Man is the dead-end host for which of the following infections?

Rabies, Tetanus. ### Explanation **1. Understanding the Concept: Dead-end Host** A **dead-end host** (or incidental host) is an organism from which an infectious agent cannot be transmitted to another susceptible host. This usually occurs because the pathogen does not reach a sufficient concentration in the blood (viremia/bacteremia) or tissues to be picked up by a vector or shed into the environment to continue its life cycle. **2. Why Option D is Correct** * **Rabies:** Humans are infected through the bite of a rabid animal (usually a dog). While the virus travels to the human brain, humans do not typically bite other humans or animals to transmit the virus further. Thus, the chain of transmission ends with the human. * **Tetanus:** *Clostridium tetani* spores enter the body through contaminated wounds. The bacteria produce toxins locally; however, tetanus is **not communicable** from person to person. The bacteria are not shed from the patient to infect others. **3. Why Other Options are Incorrect** * **Measles (Options A & B):** Humans are the **only reservoir** and the primary source of infection. It is highly contagious via respiratory droplets. Humans are definitely not dead-end hosts here. * **Yellow Fever (Options B & C):** In the **Urban Cycle**, humans serve as a reservoir where the *Aedes aegypti* mosquito picks up the virus from an infected person and transmits it to another. Therefore, humans are part of the active transmission cycle. **4. NEET-PG High-Yield Pearls** * **Other Dead-end Hosts:** Humans are also dead-end hosts for **Japanese Encephalitis (JE)**, **Hydatid Disease** (*Echinococcus granulosus*), and **Trichinellosis**. * **JE Exception:** In Japanese Encephalitis, the **pig** is the "amplifier host," and the **Ardeid bird** is the "natural reservoir," while the human is the dead-end host. * **Tetanus Key Fact:** It is the only infectious disease that is **non-communicable** but vaccine-preventable.

Q: What is the total osmolarity of reduced osmolarity ORS?

245 mOsm/L. **Explanation:** The correct answer is **245 mOsm/L**. In 2004, the WHO and UNICEF released the revised formula for **Reduced Osmolarity ORS** to improve the management of non-cholera diarrhea in children. **1. Why 245 mOsm/L is correct:** The standard ORS used previously (311 mOsm/L) was found to occasionally cause hypernatremia and increased stool output due to its high osmotic load. The reduced osmolarity version (245 mOsm/L) lowers the concentrations of Sodium and Glucose. This reduction decreases stool output by 20%, reduces vomiting by 30%, and minimizes the need for unscheduled IV fluids by 33%, making it safer and more effective. **2. Analysis of Incorrect Options:** * **A. 270 mOsm/L:** This is a common distractor; it does not represent any standard WHO ORS formulation. * **C. 290 mOsm/L:** This is the approximate osmolarity of normal plasma, but not the specific value for the WHO reduced osmolarity ORS. * **D. 310 mOsm/L:** This refers to the **Old/Standard WHO ORS** (specifically 311 mOsm/L), which is no longer the primary recommendation for routine diarrhea management. **High-Yield Clinical Pearls for NEET-PG:** * **Composition of Reduced ORS (per Liter):** * Sodium Chloride: 2.6 g * Glucose (Anhydrous): 13.5 g * Potassium Chloride: 1.5 g * Trisodium Citrate: 2.9 g * **Molar Concentrations (mmol/L):** Sodium (75), Glucose (75), Chloride (65), Potassium (20), Citrate (10). **Total = 245.** * **Sodium:Glucose Ratio:** Always **1:1** in the new formula to optimize the SGLT-1 co-transport mechanism in the small intestine. * **Trisodium Citrate** is preferred over Bicarbonate because it increases the shelf life of the ORS packet.

Q: What is the standard dose of purified protein derivative (PPD) for the Mantoux test?

5 TU. ### Explanation **Correct Option: A (5 TU)** The standard Mantoux test (Tuberculin Skin Test) utilizes **5 Tuberculin Units (TU)** of Purified Protein Derivative (PPD). In the standard procedure, 0.1 ml of PPD-S (Standard) or its equivalent (e.g., PPD RT23 with Tween 80) is injected **intradermally** on the volar aspect of the forearm using a tuberculin syringe. This dose is calibrated to provide high sensitivity and specificity for detecting *Mycobacterium tuberculosis* infection while minimizing non-specific cross-reactions. **Why Incorrect Options are Wrong:** * **B (10 TU):** While 10 TU was historically used in some regions or for specific research protocols, it is not the current global standard recommended by the WHO or the National Tuberculosis Elimination Program (NTEP). Using 10 TU increases the risk of false-positive results due to cross-reactivity with non-tuberculous mycobacteria (NTM). * **C & D (15 TU & 20 TU):** These doses are excessively high. They would lead to significant local tissue necrosis and severe inflammatory reactions, making the test results uninterpretable and clinically unsafe. **High-Yield Clinical Pearls for NEET-PG:** * **Reading the Test:** Results must be read **48 to 72 hours** after administration. * **Measurement:** Only the **induration** (palpable hardness) is measured transverse to the long axis of the arm, not the erythema (redness). * **Interpretation (NTEP India):** An induration of **≥10 mm** is generally considered positive. In HIV-positive individuals or severely malnourished children, **≥5 mm** is considered positive. * **False Negatives:** Can occur in miliary TB, malnutrition, sarcoidosis, and viral infections like Measles or HIV (Anergy). * **BCG Vaccine:** Prior BCG vaccination can cause a false positive, though the reaction usually wanes after several years.

Q: What is the true infectivity period of hepatitis A?

2-3 weeks before and 1 week after the onset of jaundice. **Explanation:** The infectivity of Hepatitis A Virus (HAV) is closely linked to the concentration of the virus excreted in the feces. The maximum viral shedding occurs during the late incubation period and the prodromal (pre-icteric) phase. **1. Why Option D is Correct:** According to standard epidemiological data (Park’s PSM), the period of maximum infectivity for Hepatitis A begins **2 to 3 weeks before** the onset of jaundice and continues for about **1 week after** the appearance of jaundice. By the time jaundice is clinically apparent, the viral titer in the stool begins to decline rapidly, and most patients are non-infectious by the end of the first week of the icteric phase. **2. Analysis of Incorrect Options:** * **Options A & C:** These suggest a 2-week post-jaundice infectivity period. While viral RNA can sometimes be detected longer, the "true infectivity" (the period where transmission is epidemiologically significant) typically ceases after 7 days of jaundice. * **Option B:** This underestimates the pre-icteric infectivity. The virus starts shedding significantly as early as 2-3 weeks before symptoms appear, making this the most dangerous period for community transmission. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mode of Transmission:** Primarily feco-oral; common in areas with poor sanitation. * **Secondary Attack Rate (SAR):** High among household contacts (approx. 20%). * **Diagnosis:** IgM anti-HAV is the gold standard for acute infection. * **Vaccination:** Live attenuated (H2 strain) or Inactivated vaccines are available. * **Post-exposure Prophylaxis:** Best given within 2 weeks of exposure (Vaccine or Immunoglobulin). * **Key Fact:** Hepatitis A does **not** cause chronic infection or a carrier state.

Question 1

Which of the following is NOT spread by Aedes mosquito?

Accepted Answer

Japanese encephalitis

Answer

Dengue fever

Answer

Chikungunya

Answer

Yellow fever

Question 2

Which of the following is NOT a characteristic of Reduced Osmolarity Oral Rehydration Solution (ORS)?

Accepted Answer

Chloride 45 mmol/litre

Answer

Sodium 75 mmol/litre

Answer

Potassium 20 mmol/litre

Answer

Osmolarity 245 mmol/litre

Question 3

Which is the main vector of malaria in urban areas?

Accepted Answer

Anopheles stephensi

Answer

Anopheles culicifacies

Answer

Anopheles fluviatilis

Answer

Anopheles epiroticus

Question 4

What is the main period of communicability of whooping cough?

Accepted Answer

Catarrhal stage

Answer

Incubation period

Answer

Paroxysmal stage

Answer

Convalescent stage

Question 5

Man is the dead-end host for which of the following infections?

Accepted Answer

Rabies, Tetanus

Answer

Tetanus, Measles

Answer

Measles, Yellow fever

Answer

Tetanus, Yellow fever

Question 6

Management of non-immunized diphtheria contacts includes all of the following except:

Accepted Answer

Prophylactic penicillin, daily throat examinations, and throat swab culture

Answer

Prophylactic penicillin and throat swab culture

Answer

Prophylactic penicillin and daily throat examinations

Answer

Single dose of toxoid and throat swab culture

Question 7

What is the total osmolarity of reduced osmolarity ORS?

Accepted Answer

245 mOsm/L

Answer

270 mOsm/L

Answer

290 mOsm/L

Answer

310 mOsm/L

Question 8

What is the standard dose of purified protein derivative (PPD) for the Mantoux test?

Accepted Answer

5 TU

Answer

10 TU

Answer

15 TU

Answer

20 TU

Question 9

An adult male patient presented with cough and fever for 3 months and intermittent haemoptysis. His sputum was positive for AFB. He had previously received 3 weeks of RHZE treatment from a nearby hospital before discontinuing it. How will you categorize and manage this patient?

Accepted Answer

Category I, treatment regimen (RHZ) for 3 months

Answer

Category III, treatment regimen (RHZ) for 3 months

Answer

Category II, treatment regimen (RHZ) for 3 months

Answer

Category II, treatment regimen (RHZ) for 3 months

Question 10

What is the true infectivity period of hepatitis A?

Accepted Answer

2-3 weeks before and 1 week after the onset of jaundice

Answer

1-2 weeks before and 2 weeks after the onset of jaundice

Answer

1-2 weeks before and 1 week after the onset of jaundice

Answer

2-3 weeks before and 2 weeks after the onset of jaundice

Infectious Diseases — MCQs

Infectious Diseases — MCQs

On this page

Practice by Chapter

Want unlimited practice?