Infectious Diseases — MCQs

A 62-year-old man with a history of mild intermittent bronchial asthma presents for a first visit after not seeing a doctor for over 10 years. He is sexually active with a single long-term partner and does not recall receiving any vaccines since childhood. Which of the following vaccines should be offered?

Q609Medium

A patient is diagnosed with HIV and Tuberculosis. What is the recommended order for starting Anti-Tuberculosis Therapy (ATT) and combination Antiretroviral Therapy (cART)?

Q610Medium

Which of the following can be used as a diagnostic modality for herpes labialis?

Infectious Diseases Indian Medical PG Practice Questions and MCQs

Question 601: In most cases, the only organism responsible for all allergic symptoms and signs of ABPA is:

A. Aspergillus tereus
B. Aspergillus flavus
C. Aspergillus fumigatus (Correct Answer)
D. Aspergillus nidularis

Explanation: **Explanation:** **Allergic Bronchopulmonary Aspergillosis (ABPA)** is a complex hypersensitivity reaction (Type I, III, and IV) to the colonization of the airways by *Aspergillus* species. **Why the Correct Answer is Right:** While several fungi can cause ABPA-like syndromes, **Aspergillus fumigatus** is the predominant organism responsible for the vast majority of clinical cases [1]. It thrives at body temperature and possesses specific proteases and allergens (like Asp f1 to f6) that trigger an intense immune response in predisposed individuals (typically those with Asthma or Cystic Fibrosis). This leads to eosinophilic inflammation, elevated IgE levels, and characteristic "finger-in-glove" mucoid impaction. **Why the Incorrect Options are Wrong:** * **Aspergillus flavus:** While it is a common cause of fungal sinusitis and keratitis, it is rarely the primary driver of ABPA. * **Aspergillus terreus:** Known for its inherent resistance to Amphotericin B, it primarily causes invasive aspergillosis in immunocompromised hosts rather than allergic syndromes [2]. * **Aspergillus nidulans:** This species is more commonly associated with infections in patients with Chronic Granulomatous Disease (CGD) and is an infrequent cause of ABPA. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnostic Criteria (ISHAM):** Predisposing conditions (Asthma/CF) + Elevated total IgE (>1000 IU/mL) + Positive skin test/IgE for *A. fumigatus*. * **Radiology:** Central bronchiectasis (classic finding) [1] and "High Attenuation Mucus" (HAM) on CT is highly specific. * **Treatment:** Oral Corticosteroids (to suppress inflammation) and Itraconazole (to reduce fungal burden). * **Key Lab Finding:** Peripheral blood eosinophilia (>500 cells/µL).

Question 602: Which valve is most commonly affected in infective endocarditis associated with intravenous drug use?

A. Aortic valve
B. Mitral valve
C. Tricuspid valve (Correct Answer)
D. Pulmonary valve

Explanation: ### Explanation **Correct Option: C. Tricuspid valve** In the general population, the mitral valve is the most common site for infective endocarditis (IE). However, **intravenous drug use (IVDU)** significantly alters this epidemiology. In IVDU patients, the **tricuspid valve** is the most frequently affected (involved in >50% of cases). **Pathophysiology:** The preference for right-sided valves in IVDU is attributed to the direct entry of pathogens and particulate matter (used to bulk up drugs) into the venous circulation [1]. These particles cause micro-trauma to the tricuspid endothelium, creating a nidus for bacterial attachment [3]. *Staphylococcus aureus* is the most common causative organism in this demographic [1]. **Analysis of Incorrect Options:** * **A. Aortic valve:** While the aortic valve is the second most common site in the general population, it is less frequently involved than the tricuspid valve in the specific context of IVDU. * **B. Mitral valve:** This is the most common site for IE overall (non-IVDU), often associated with underlying mitral valve prolapse or rheumatic heart disease. * **C. Pulmonary valve:** Although it is a right-sided valve, it is the least commonly affected valve in IE, even among IVDU patients. **High-Yield Clinical Pearls for NEET-PG:** * **Most common organism in IVDU IE:** *Staphylococcus aureus* (often MRSA) [1]. * **Septic Pulmonary Emboli:** A classic complication of tricuspid IE, presenting as multiple "fleur-de-lis" or cavitary lesions on a chest X-ray. * **Murmur:** Right-sided murmurs (like tricuspid regurgitation) typically **increase in intensity during inspiration** (Carvallo's sign). * **Culture-Negative IE:** Most commonly due to prior antibiotic use or fastidious organisms like the **HACEK** group or *Coxiella burnetii*. * **Diagnosis:** Diagnosis is confirmed using the Duke Criteria, where IVDU is considered a predisposing minor criterion [2].

Question 603: Which antiretroviral drug is most commonly associated with nephrolithiasis?

A. Indinavir (Correct Answer)
B. Zidovudine
C. Tenofovir
D. Eirenz

Explanation: **Explanation:** **Indinavir** (a Protease Inhibitor) is the correct answer because it is notorious for causing **nephrolithiasis** (kidney stones). The underlying mechanism is the drug's poor solubility at physiological urinary pH, leading to the formation of indinavir crystals in the collecting ducts. Approximately 4–10% of patients taking Indinavir develop symptomatic stones, which are typically radiolucent on X-ray but visible on CT scans. Management involves aggressive hydration (at least 1.5 liters of water daily) to prevent crystal precipitation. **Analysis of Incorrect Options:** * **Zidovudine (AZT):** A Nucleoside Reverse Transcriptase Inhibitor (NRTI) primarily associated with **bone marrow suppression** (anemia, neutropenia) and myopathy [1]. It does not cause renal stones. * **Tenofovir (TDF):** While Tenofovir is nephrotoxic, it typically causes **Fanconi Syndrome** (proximal renal tubular acidosis) or acute kidney injury, rather than nephrolithiasis. * **Efavirenz:** A Non-Nucleoside Reverse Transcriptase Inhibitor (NNRTI) most famous for **CNS side effects**, such as vivid dreams, dizziness, and neuropsychiatric symptoms. **High-Yield Clinical Pearls for NEET-PG:** * **Indinavir:** Associated with "Crixivan belly" (lipodystrophy) and hyperbilirubinemia (indirect). * **Tenofovir:** Monitor Serum Creatinine and urine glucose (for Fanconi syndrome). * **Nevirapine:** Most common ARV to cause Stevens-Johnson Syndrome (SJS) and hepatotoxicity. * **Abacavir:** Associated with a hypersensitivity reaction linked to the **HLA-B*5701** allele. * **Atazanavir:** Another Protease Inhibitor that can cause nephrolithiasis and hyperbilirubinemia, but Indinavir remains the classic association in exams.

Question 604: What is the best diagnostic modality for rotavirus diarrhea?

A. Isolation of virus from stool culture (Correct Answer)
B. Detection of antigen in serum
C. Detection of antigen in stool
D. Detection of antibody in serum

Explanation: **Explanation:** **Rotavirus** is the most common cause of severe, dehydrating diarrhea in infants and young children worldwide. **Why Option A is Correct:** While several methods exist for diagnosing Rotavirus, the **isolation of the virus from stool** (typically via electron microscopy or cell culture) remains the "gold standard" for definitive identification. In the context of academic examinations like NEET-PG, when asked for the "best" or most definitive modality, isolation/visualization of the viral particles from the primary site of infection (stool) is prioritized. **Analysis of Incorrect Options:** * **Option B (Antigen in serum):** Rotavirus is a localized intestinal infection; viremia is rare and clinically insignificant for diagnosis. Therefore, serum antigen testing is not used. * **Option C (Antigen in stool):** Enzyme Immunoassays (EIA) and Latex Agglutination to detect VP6 antigen in stool are the **most commonly used** tests in clinical practice because they are rapid and inexpensive. However, they are generally considered "screening" or "rapid" tests rather than the definitive "best" modality compared to isolation. * **Option D (Antibody in serum):** Serology (detecting IgA or IgG) is primarily used for retrospective epidemiological studies and is not helpful for diagnosing an acute episode of diarrhea. **High-Yield NEET-PG Pearls:** * **Mechanism:** Rotavirus produces an enterotoxin called **NSP4**, which induces secretory diarrhea by increasing intracellular calcium. * **Seasonality:** In temperate climates, it is a "winter diarrhea." * **Vaccines:** Rotavirus vaccines (Rotarix, RotaTeq, Rotavac) are live-attenuated and administered orally. * **Morphology:** It belongs to the *Reoviridae* family, featuring a double-stranded RNA (dsRNA) genome and a characteristic wheel-like appearance under electron microscopy (*Rota* = wheel).

Question 605: Which of the following is the most common symptom of tetanus?

A. Risus sardonicus
B. Trismus (Correct Answer)
C. Hyperreflexia
D. Tonic clonic convulsion

Explanation: **Explanation:** **Trismus (Lockjaw)** is the most common and typically the earliest presenting symptom of generalized tetanus [1]. It is caused by the masseter muscle's hypersensitivity to the tetanus toxin (tetanospasmin), which blocks the release of inhibitory neurotransmitters (GABA and glycine) in the spinal cord and brainstem [1]. This leads to sustained muscular contraction. In over 50–75% of cases, trismus is the hallmark initial sign that leads patients to seek medical attention. **Analysis of Incorrect Options:** * **Risus sardonicus:** While a characteristic sign of tetanus, it refers to the "sardonic smile" caused by the contraction of the facial muscles (frontalis and orbicularis oris). It usually occurs *after* or alongside trismus, but is not as universally the first or most common symptom. * **Hyperreflexia:** This is a clinical finding in tetanus due to the loss of upper motor neuron inhibition [1], but it is a non-specific sign and not the primary presenting complaint. * **Tonic-clonic convulsions:** Tetanus causes "tetanic spasms" (generalized muscle contractions), which are distinct from true epileptic tonic-clonic seizures. These spasms are triggered by external stimuli (noise, light) and occur in more advanced stages of the disease. **Clinical Pearls for NEET-PG:** * **Pathophysiology:** Tetanospasmin travels via **retrograde axonal transport** to the CNS [1]. * **Cephalic Tetanus:** A rare form involving cranial nerves (most commonly **CN VII**); it can occasionally present with flaccid paralysis instead of spasm. * **Neonatal Tetanus:** Usually occurs due to umbilical cord infection; the first sign is typically the **inability to suckle**. * **Management:** The priority is neutralizing unbound toxin with **Human Tetanus Immunoglobulin (HTIG)** and wound debridement. Metronidazole is the preferred antibiotic.

Question 606: All of the following statements about miliary tuberculosis are true except?

A. May occur following primary infection
B. May occur following secondary reactivation
C. Sputum microscopy is usually negative
D. Mantoux test is always positive (Correct Answer)

Explanation: Explanation: Miliary tuberculosis (TB) is a life-threatening form of TB resulting from the **hematogenous dissemination** of *Mycobacterium tuberculosis* [1]. **Why Option D is the Correct Answer (The "Except" Statement):** The Mantoux (Tuberculin Skin Test) is **not** always positive in miliary TB. In fact, it is negative in approximately **30–50%** of cases. This occurs due to **anergy**, where the body’s cell-mediated immune response is overwhelmed by the high bacterial load or suppressed by severe systemic illness, malnutrition, or concurrent viral infections (like HIV). **Analysis of Other Options:** * **Option A & B:** Miliary TB can occur during **primary infection** (common in children) or as a result of **secondary reactivation** of a latent focus (common in adults/elderly) [1]. Both represent a failure of the immune system to contain the bacilli locally. * **Option C:** Sputum microscopy is **usually negative** because miliary TB involves interstitial seeding rather than cavitary lesions that communicate with the airways. Diagnosis often requires biopsy (liver/bone marrow) or high-resolution CT (HRCT). **NEET-PG High-Yield Pearls:** * **Chest X-ray:** Classically shows 1–2 mm "millet seed" sized spots distributed uniformly throughout both lung fields [1]. * **Fundoscopy:** The presence of **choroid tubercles** is a pathognomonic clinical sign of miliary TB [1]. * **Common Lab Finding:** Hyponatremia (often due to SIADH or adrenal involvement). * **Gold Standard for Diagnosis:** While CXR is the initial tool, **transbronchial lung biopsy** or bone marrow biopsy has a higher diagnostic yield.

Question 607: Which of the following is the most common cause for fulminant hepatitis?

A. Hepatitis A
B. Hepatitis B
C. Hepatitis C
D. Hepatitis D (Correct Answer)

Explanation: **Explanation:** Fulminant hepatitis (Acute Liver Failure) is defined as the rapid onset of hepatic encephalopathy and coagulopathy in a patient without pre-existing cirrhosis [1]. **Why Hepatitis D is the correct answer:** Hepatitis D Virus (HDV) is a defective RNA virus that requires the presence of Hepatitis B (HBsAg) to replicate. While Hepatitis B is the most common cause of fulminant hepatitis in terms of absolute numbers globally, **Hepatitis D carries the highest risk (incidence rate) of progressing to fulminant failure.** Specifically, a **superinfection** (HDV infection in a chronic HBV carrier) leads to fulminant hepatitis in approximately 5–20% of cases, a significantly higher proportion than any other hepatitis virus. **Analysis of Incorrect Options:** * **Hepatitis A:** While it can cause fulminant failure, it occurs in less than 1% of cases, primarily in older adults or those with underlying liver disease [3]. * **Hepatitis B:** HBV is a common cause of fulminant hepatitis (especially in Asia), but the percentage of acute HBV cases progressing to fulminant failure is low (~1%). * **Hepatitis C:** HCV is a major cause of chronic liver disease but is **extremely rare** as a cause of acute fulminant hepatitis [4]. **High-Yield Clinical Pearls for NEET-PG:** * **Most common cause of fulminant hepatitis in pregnancy:** Hepatitis E (especially in the 3rd trimester, with mortality rates up to 20%). * **Coinfection vs. Superinfection:** Coinfection (HBV + HDV simultaneously) usually results in acute hepatitis similar to HBV alone. Superinfection (HDV on top of chronic HBV) is what leads to severe, fulminant disease and rapid progression to cirrhosis. * **Drug-induced:** Globally, Acetaminophen (Paracetamol) toxicity is the most common non-viral cause of fulminant hepatic failure [2].

Question 608: A 62-year-old man with a history of mild intermittent bronchial asthma presents for a first visit after not seeing a doctor for over 10 years. He is sexually active with a single long-term partner and does not recall receiving any vaccines since childhood. Which of the following vaccines should be offered?

A. Pneumococcal, influenza, zoster, and tetanus-diphtheria-acellular pertussis (Tdap) (Correct Answer)
B. Pneumococcal, influenza, zoster, and tetanus-diphtheria (Td)
C. Pneumococcal, influenza, and human papillomavirus (HPV)
D. Pneumococcal, influenza, and tetanus-diphtheria-acellular pertussis (Tdap)

Explanation: This question tests the application of adult immunization schedules based on age and underlying comorbidities. **Explanation of the Correct Answer (A):** The patient is 62 years old with a history of bronchial asthma. The recommendation for vaccines is based on the following: 1. **Pneumococcal Vaccine:** Patients with chronic lung disease (including asthma) should receive the pneumococcal vaccine regardless of age. 2. **Influenza Vaccine:** Recommended annually for all adults, especially those with chronic respiratory conditions. 3. **Zoster Vaccine:** The recombinant zoster vaccine (Shingrix) is recommended for all immunocompetent adults aged **≥50 years** (two doses). 4. **Tdap:** All adults should receive a one-time dose of Tdap (to provide protection against pertussis) if they haven't received it before, followed by Td or Tdap boosters every 10 years. Since he hasn't seen a doctor in 10 years, he is due for a booster. **Analysis of Incorrect Options:** * **Option B:** Incorrect because **Tdap** is preferred over Td for at least one dose in adulthood to ensure immunity against *Bordetella pertussis*. * **Option C:** Incorrect because it misses Zoster and Tdap. **HPV** vaccine is generally recommended only up to age 26 (though shared decision-making can occur up to age 45; it is not indicated for a 62-year-old). * **Option D:** Incorrect because it omits the **Zoster** vaccine, which is a standard recommendation for his age group (≥50). **High-Yield Clinical Pearls for NEET-PG:** * **Zoster Vaccine:** Age threshold is **50 years** for Shingrix (recombinant). * **Pneumococcal Vaccine:** Indicated <65 years if the patient has DM, chronic heart/lung/liver disease, or smokes. All adults receive it at **≥65 years**. * **Tdap vs. Td:** Tdap is given once in a lifetime (usually replacing one Td booster) to prevent the transmission of pertussis to infants. * **Live Vaccines:** Avoid in pregnancy and severely immunocompromised states (e.g., MMR, Varicella, Yellow Fever).

Question 609: A patient is diagnosed with HIV and Tuberculosis. What is the recommended order for starting Anti-Tuberculosis Therapy (ATT) and combination Antiretroviral Therapy (cART)?

A. Start ATT first (Correct Answer)
B. Start cART first
C. Start both simultaneously
D. Start cART only

Explanation: **Explanation:** In patients co-infected with HIV and Tuberculosis (TB), the standard of care is to **start Anti-Tuberculosis Therapy (ATT) first**, followed by combination Antiretroviral Therapy (cART). **Why ATT is started first:** The primary goal is to reduce the mycobacterial load and stabilize the patient before introducing antiretrovirals. Starting cART too early or simultaneously increases the risk of **Immune Reconstitution Inflammatory Syndrome (IRIS)**. IRIS occurs when the recovering immune system mounts an exaggerated inflammatory response against TB antigens, leading to clinical worsening. Furthermore, starting both together complicates the management of drug-drug interactions (especially with Rifampicin) and overlapping toxicities (like hepatotoxicity). **Analysis of Incorrect Options:** * **B & C:** Starting cART first or simultaneously significantly raises the risk of paradoxical IRIS and increases the pill burden, which may lead to poor adherence and severe adverse drug reactions. * **D:** Treating HIV alone while ignoring active TB is fatal. TB is a leading cause of mortality in HIV patients; uncontrolled TB accelerates HIV progression by increasing viral replication. **Clinical Pearls for NEET-PG:** * **Timing:** cART should generally be started **within 2 weeks** if the CD4 count is **<50 cells/mm³** and within **8 weeks** for others. * **Exception:** In **TB Meningitis**, cART should be delayed (usually 6–8 weeks) regardless of CD4 count to prevent life-threatening intracranial IRIS. * **Drug Interaction:** Rifampicin is a potent enzyme inducer. When used with cART, **Efavirenz** is often preferred, or the dose of **Dolutegravir** must be increased to twice daily.

Question 610: Which of the following can be used as a diagnostic modality for herpes labialis?

A. Tzanck smear
B. Culture
C. Gram stain under microscope
D. PCR assay (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. PCR assay** **1. Why PCR assay is the correct answer:** Polymerase Chain Reaction (PCR) is currently the **gold standard** and the most sensitive diagnostic modality for detecting Herpes Simplex Virus (HSV-1 and HSV-2). In cases of herpes labialis, PCR can detect viral DNA even when the viral load is low or during the crusting stage of the lesion. It is significantly more sensitive than viral culture and provides rapid results, making it the preferred choice in modern clinical practice. **2. Analysis of Incorrect Options:** * **A. Tzanck smear:** While historically popular, it is a non-specific test. It identifies **multinucleated giant cells** and Cowdry Type A inclusion bodies, which are seen in both HSV and Varicella-Zoster Virus (VZV). It cannot differentiate between the two, nor can it identify the specific HSV strain. * **B. Culture:** Viral culture was previously the gold standard. However, it has a high false-negative rate because the virus is labile and requires viable cells. Its sensitivity decreases rapidly as the lesion begins to heal. * **C. Gram stain:** This is used for identifying **bacteria** based on their cell wall properties. It has no utility in diagnosing viral infections like Herpes. **3. Clinical Pearls for NEET-PG:** * **Most common site:** The vermilion border of the lip is the most frequent site for herpes labialis (HSV-1) [1]. * **Histopathology:** Look for "Acantholysis" and "Margination of chromatin" in Tzanck smears. * **Treatment of choice:** Oral Acyclovir or Valacyclovir. * **Recurrence:** HSV remains latent in the **Trigeminal ganglion** for orofacial herpes and the **Sacral ganglion** for genital herpes [1]. * **Encephalitis:** HSV-1 is the most common cause of sporadic fatal encephalitis; here, **CSF PCR** is the diagnostic investigation of choice.

Practice by Chapter

Principles of Antimicrobial Therapy

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Fever of Unknown Origin

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HIV/AIDS and Related Infections

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Tuberculosis and Mycobacterial Diseases

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Tropical and Parasitic Infections

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Viral Infections (Hepatitis, Herpes, etc.)

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Healthcare-Associated Infections

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Fungal Infections

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Sepsis and Septic Shock

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Infection in Immunocompromised Hosts

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Emerging and Re-emerging Infections

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Antimicrobial Resistance

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Vaccination Principles

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