Infectious Diseases — MCQs

Infectious Diseases Indian Medical PG Practice Questions and MCQs

Question 561: In a patient with active chronic hepatitis B, which of the following is NOT typically seen?

A. HBsAg
B. IgM anti-HBcAg (Correct Answer)
C. HBeAg
D. Anti-HBsAg

Explanation: ### Explanation The key to answering this question lies in distinguishing between **acute** and **chronic** Hepatitis B virus (HBV) infection based on serological markers [2]. **Why IgM anti-HBcAg is the correct answer:** **IgM anti-HBc (Immunoglobulin M antibody to Hepatitis B core antigen)** is the hallmark of **acute infection** [1]. It appears shortly after HBsAg and is the only marker present during the "window period." In chronic hepatitis B (defined as the persistence of HBsAg for >6 months), the immune response shifts from IgM to **IgG anti-HBc** [2]. Therefore, IgM anti-HBc is typically absent in chronic cases, except during rare, severe acute flares. **Analysis of Incorrect Options:** * **A. HBsAg (Hepatitis B Surface Antigen):** This is the first marker to appear and its persistence for more than 6 months is the defining diagnostic criterion for chronic HBV infection [2]. * **C. HBeAg (Hepatitis B e-Antigen):** This indicates active viral replication and high infectivity. It is commonly seen in the "Immune Tolerant" and "Immune Active" phases of chronic HBV [2]. * **D. Anti-HBsAg:** While usually a sign of recovery or vaccination, low levels of Anti-HBs can coexist with HBsAg in about 10–25% of chronic carriers [2]. These antibodies are unable to neutralize the virus, so their presence does not rule out a chronic state. **NEET-PG High-Yield Pearls:** 1. **Window Period:** The interval where HBsAg disappears but Anti-HBs has not yet appeared. **IgM anti-HBc** is the diagnostic marker here [1]. 2. **Chronic Infection Marker:** **IgG anti-HBc** persists for life in anyone who has had a natural infection (both recovered and chronic) [2]. 3. **Vaccination Marker:** A vaccinated individual will be **Anti-HBs positive** but **Anti-HBc negative** (since the vaccine contains only the surface protein) [2]. 4. **Best indicator of viral replication:** HBV DNA levels (Quantitative PCR) [1].

Question 562: Which Mycobacterium species is the most common cause of tuberculous infection in patients with AIDS?

A. M. Avium Intracellulare (Correct Answer)
B. M. Scrofulaceum
C. M. Ulcerans
D. M. Tuberculosis

Explanation: In patients with AIDS, the risk of opportunistic infections increases as the CD4+ T-cell count declines. While *M. tuberculosis* can occur at any CD4 level [1], **Mycobacterium Avium-Intracellulare Complex (MAC)** is the most common **Non-Tuberculous Mycobacteria (NTM)** causing disseminated infection in advanced HIV/AIDS, typically when the **CD4 count falls below 50 cells/mm³** [2]. * **M. Avium Intracellulare (Option A):** This is the correct answer. MAC is ubiquitous in the environment (soil/water). In AIDS patients, it presents as a disseminated multi-organ disease characterized by fever, night sweats, weight loss, abdominal pain, and profound anemia [2]. * **M. Scrofulaceum (Option B):** This species is primarily associated with **cervical lymphadenitis** (scrofula) in children and is rarely the cause of disseminated disease in AIDS. * **M. Ulcerans (Option C):** This is the causative agent of **Buruli ulcer**, a chronic necrotizing skin and soft tissue infection. It is not a common opportunistic pathogen in HIV. * **M. Tuberculosis (Option D):** While *M. tuberculosis* is the most common cause of "tuberculosis" globally and is highly prevalent in HIV patients [1], the question specifically targets the most common atypical/opportunistic mycobacterial infection associated with the immunocompromised state of AIDS. In the context of NTMs and late-stage AIDS, MAC is the classic association [2]. **High-Yield Clinical Pearls for NEET-PG:** * **Prophylaxis:** Azithromycin or Clarithromycin is indicated for MAC prophylaxis in HIV patients with CD4 < 50 cells/mm³. * **Diagnosis:** MAC is best diagnosed via **blood culture** (using BACTEC bottles) or bone marrow biopsy. * **Treatment:** The preferred regimen is Clarithromycin + Ethambutol (+/- Rifabutin). * **Biopsy Finding:** MAC often shows "foamy macrophages" filled with Acid-Fast Bacilli (AFB).

Question 563: Which of the following conditions is NOT considered an AIDS-defining illness?

A. Oral candidiasis
B. Invasive carcinoma of the cervix (Correct Answer)
C. Toxoplasmosis of the central nervous system
D. Kaposi sarcoma

Explanation: The correct answer is **A. Oral candidiasis**. *Note: There appears to be a discrepancy in the provided prompt's marking. According to the CDC and WHO classification of HIV/AIDS, **Invasive Cervical Carcinoma IS an AIDS-defining illness**, whereas **Oral Candidiasis is NOT** [1].* **1. Why Oral Candidiasis is the correct answer:** Oral candidiasis (thrush) is a common opportunistic infection in HIV patients, typically occurring when CD4 counts fall below 200-500 cells/mm³. However, it is classified as a **Category B (Symptomatic, non-AIDS defining)** condition [1], [2]. In contrast, **Esophageal candidiasis** (candidiasis of the esophagus, trachea, or lungs) is considered a Category C (AIDS-defining) illness [1]. **2. Why the other options are AIDS-defining (Category C):** * **Invasive Carcinoma of the Cervix:** Added to the CDC list in 1993, this is one of the three AIDS-defining malignancies (along with Kaposi Sarcoma and Non-Hodgkin Lymphoma) [1]. * **Toxoplasmosis of the CNS:** A classic AIDS-defining neurological infection, typically seen when CD4 counts drop below 100 cells/mm³ [3]. * **Kaposi Sarcoma:** Caused by HHV-8, this is the most common malignancy associated with HIV and is a hallmark AIDS-defining condition [4]. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for AIDS-defining Malignancies:** **C**ervical (Invasive), **K**aposi Sarcoma, **L**ymphoma (Burkitt’s, Immunoblastic, or Primary CNS) [1], [3]. * **CD4 Thresholds:** * <200: *Pneumocystis jirovecii* pneumonia (PCP). * <100: Toxoplasmosis, Cryptococcosis [3]. * <50: Mycobacterium avium complex (MAC), CMV Retinitis. * **Oral Hairy Leukoplakia** (caused by EBV) is also a Category B condition, not AIDS-defining.

Question 564: Mucosal invasion of the intestine causes which type of diarrhea?

A. Watery diarrhea
B. Rice stool
C. Dysentery (Correct Answer)
D. None

Explanation: **Explanation:** The correct answer is **Dysentery**. **1. Understanding the Mechanism (Why Dysentery is correct):** Diarrhea is broadly classified into secretory (watery) and inflammatory (invasive). When pathogens (such as *Shigella*, *Entamoeba histolytica*, or Enteroinvasive *E. coli*) invade the intestinal mucosa, they cause direct cellular damage, inflammation, and ulceration [1]. This process results in the release of blood, mucus, and inflammatory cells (pus) into the intestinal lumen. This clinical presentation—small-volume stools containing blood and mucus, often accompanied by tenesmus (straining) and fever—is defined as **dysentery** [1]. **2. Analysis of Incorrect Options:** * **Watery diarrhea (Option A):** This is typically caused by non-invasive pathogens (e.g., *Vibrio cholerae*, ETEC) or viruses (e.g., Rotavirus) [1]. These organisms produce toxins that alter electrolyte transport (secretory) or cause osmotic imbalances without destroying the mucosal architecture. * **Rice stool (Option B):** This is a specific subtype of profound watery diarrhea characteristic of **Cholera**. It is caused by the *Cholera toxin* acting on adenylate cyclase, leading to massive secretion of water and electrolytes without mucosal invasion [3]. **3. NEET-PG Clinical Pearls:** * **Site of Involvement:** Invasive diarrhea (Dysentery) usually involves the **large intestine (colon)**, whereas watery diarrhea usually involves the **small intestine**. * **Common Pathogens:** * *Bacillary Dysentery:* Most common cause is **Shigella** (specifically *S. sonnei* in developed and *S. dysenteriae* in developing regions). * *Amoebic Dysentery:* Caused by **Entamoeba histolytica** (look for "flask-shaped ulcers" on biopsy) [2]. * **Key Distinction:** Fever is a hallmark of invasive/inflammatory diarrhea but is usually absent or low-grade in pure secretory diarrhea.

Question 565: A 78-year-old woman with a history of stroke presents with poor appetite and confusion. A chest X-ray reveals a left lower lobe infiltrate. Blood cultures are positive for S pneumoniae sensitive to penicillin. Which of the following is the most likely complication of pneumococcal pneumonia?

A. Peritonitis
B. Empyema (Correct Answer)
C. Pericarditis
D. Endocarditis

Explanation: **Explanation:** **1. Why Empyema is the Correct Answer:** *Streptococcus pneumoniae* is the most common cause of community-acquired pneumonia (CAP). The most frequent complication of pneumococcal pneumonia is a **parapneumonic effusion**, occurring in up to 40% of cases [2]. If this fluid becomes infected or develops into a loculated collection of pus, it is termed **empyema** [1]. While the incidence of empyema has decreased in the antibiotic era, it remains the most common "local" complication of the disease, especially in elderly patients with comorbidities. **2. Analysis of Incorrect Options:** * **A. Peritonitis:** While *S. pneumoniae* can cause primary peritonitis (classically in patients with nephrotic syndrome or cirrhosis), it is a rare complication of pneumonia in the general population. * **C. Pericarditis:** Purulent pericarditis is a severe but rare complication, usually occurring via direct extension from an adjacent lung focus or via hematogenous spread. * **D. Endocarditis:** Pneumococcal endocarditis is now rare (<1% of cases). It typically involves the aortic valve and is classically associated with the **Austrian Syndrome** (the triad of pneumonia, meningitis, and endocarditis). **3. NEET-PG High-Yield Pearls:** * **Most common complication overall:** Parapneumonic effusion (Empyema is the most common *purulent* complication). * **Austrian Syndrome:** Triad of *S. pneumoniae* pneumonia, meningitis, and endocarditis (usually aortic valve). * **Risk Factors:** Alcoholism, splenectomy (asplenia), and COPD are major risk factors for severe pneumococcal disease. * **Diagnosis:** The "Rusty sputum" is a classic (though not always present) clinical sign of pneumococcal pneumonia [3]. * **Vaccination:** PPSV23 (polysaccharide) and PCV13 (conjugate) are key preventive measures for the elderly and immunocompromised.

Question 566: In a patient with cerebral malaria, which of the following asexual stages of Plasmodium falciparum can be seen on peripheral smear?

A. Tropozoites (Correct Answer)
B. Schizonts
C. Gametocytes
D. Hypnozoites

Explanation: ### Explanation **Correct Answer: A. Trophozoites** In *Plasmodium falciparum* infections, the peripheral blood smear typically shows only **early trophozoites (ring forms)** and occasionally gametocytes [1]. This is due to a unique phenomenon called **sequestration**. As the parasite matures from the ring stage to the late trophozoite and schizont stages, it expresses **PfEMP-1** (Plasmodium falciparum erythrocyte membrane protein 1) on the surface of the RBC. This protein causes "knobs" on the RBC membrane, leading to **cytoadherence** (sticking to vascular endothelium) and **rosetting** (sticking to uninfected RBCs). Consequently, mature asexual stages (late trophozoite and schizonts) are sequestered in the deep capillaries of internal organs (brain, kidneys, heart), leading to microvascular obstruction and organ dysfunction, such as **cerebral malaria**. **Analysis of Incorrect Options:** * **B. Schizonts:** These are sequestered in the deep vascular beds. Their presence in a peripheral smear is rare and usually indicates a very high parasite load or a poor prognosis. * **C. Gametocytes:** These are the **sexual stages** of the parasite [1]. While they can be seen in the peripheral smear (characteristically banana/crescent-shaped), the question specifically asks for **asexual stages**. * **D. Hypnozoites:** These are dormant liver stages found only in *P. vivax* and *P. ovale* [1]. *P. falciparum* does not have a hypnozoite stage and therefore does not cause true relapses. **High-Yield NEET-PG Pearls:** * **Maurer’s dots:** Coarse granulations seen in RBCs infected with *P. falciparum*. * **Multiple rings per RBC** and **Accole/Applique forms** (parasites at the periphery of RBC) are diagnostic hallmarks of *P. falciparum*. * **Cerebral Malaria definition:** Coma (GCS <11) at least 1 hour after a seizure, with *P. falciparum* parasitemia, and no other cause of encephalopathy. * **Drug of Choice:** Intravenous **Artesunate** is the gold standard for severe/cerebral malaria.

Question 567: What are the major signs included by WHO for AIDS diagnosis?

A. Chronic diarrhea > 1 month (Correct Answer)
B. Chronic cough > 1 month
C. Chronic fever > 1 month
D. Weight loss of at least 10% of body weight

Explanation: The WHO Case Definition for AIDS (Bangui Definition) is a high-yield clinical staging tool used primarily in resource-limited settings where sophisticated laboratory testing (like CD4 counts) may not be available [1]. ### **Explanation of the Correct Answer** The WHO clinical criteria for AIDS in adults categorize symptoms into **Major** and **Minor** signs. To diagnose AIDS clinically, a patient must have at least **2 Major signs** and **1 Minor sign** in the absence of other known causes of immunosuppression. The **Major Signs** are: 1. **Weight loss > 10%** of body weight [1]. 2. **Chronic diarrhea > 1 month.** (Option A is correct). 3. **Prolonged fever > 1 month** (intermittent or constant). ### **Analysis of Incorrect Options** * **B. Chronic cough > 1 month:** This is classified as a **Minor Sign**, not a major sign. It often points toward pulmonary tuberculosis or fungal infections. * **C. Chronic fever > 1 month:** While fever is a major sign, the standard definition specifies "prolonged fever" rather than "chronic fever." However, in the context of this specific question format, Option A is the most classically cited major sign alongside weight loss. * **D. Weight loss of at least 10% of body weight:** While this is a Major Sign, Option A is often prioritized in MCQ patterns to test the specific duration (>1 month) associated with the gastrointestinal manifestation. ### **NEET-PG High-Yield Pearls** * **Minor Signs include:** Persistent cough (>1 month), generalized pruritic dermatitis, recurrent herpes zoster, oropharyngeal candidiasis, and generalized lymphadenopathy. * **The "Slim Disease":** In Africa, AIDS was historically referred to as "Slim Disease" due to the combination of profound weight loss and chronic diarrhea. * **Pediatric Criteria:** For children, the major signs include weight loss/failure to thrive, chronic diarrhea (>1 month), and chronic fever (>1 month). * **Definitive Diagnosis:** Presence of **Generalized Kaposi Sarcoma** or **Cryptococcal Meningitis** are sufficient for an AIDS diagnosis on their own.

Question 568: Miliary tuberculosis is characterized by:

A. Primary infection pattern
B. Disseminated lesions throughout multiple organs (Correct Answer)
C. Limited to organs outside the lungs
D. Not a form of tuberculosis

Explanation: **Explanation:** **Miliary Tuberculosis (TB)** is a life-threatening form of the disease caused by the **hematogenous (blood-borne) dissemination** of *Mycobacterium tuberculosis* [1]. The term "miliary" refers to the characteristic gross appearance of multiple tiny, millet-seed-sized (1–2 mm) granulomas scattered throughout various organs [1]. * **Why Option B is Correct:** Miliary TB occurs when a TB lesion erodes into a blood vessel or lymphatic duct, allowing the bacilli to spread systemically [1]. This results in the formation of synchronous, small, firm, yellowish-white lesions in multiple organs, most commonly the **lungs, liver, spleen, bone marrow, and choroid of the eye.** * **Why Option A is Incorrect:** While miliary TB can occur during primary infection (especially in children or the immunocompromised), it is not a "pattern" of primary TB itself; it is a complication of dissemination that can occur during either primary or post-primary (reactivation) stages [1]. * **Why Option C is Incorrect:** Miliary TB frequently involves the lungs (often showing a "millet-seed" pattern on Chest X-ray). It is defined by systemic spread, not by the exclusion of pulmonary involvement [1]. * **Why Option D is Incorrect:** Miliary TB is a well-recognized, severe clinical manifestation of tuberculosis. **NEET-PG High-Yield Pearls:** 1. **Chest X-ray:** Shows a classic "miliary pattern" (1–2 mm stippled opacities) but may be normal in early stages [1]. 2. **Choroid Tubercles:** Pathognomonic finding on funduscopic examination [1]. 3. **Gold Standard Diagnosis:** Often requires biopsy (liver or bone marrow) or culture if sputum is negative. 4. **Hyponatremia:** A common biochemical abnormality in miliary TB (due to SIADH or adrenal involvement). 5. **Cryptic Miliary TB:** Occurs in the elderly; presents with fever of unknown origin (FUO) and often lacks the classic radiographic pattern.

Question 569: A 45-year-old female is diagnosed as a case of pneumococcal meningitis. Her blood samples were sent for culture sensitivity. In the meantime, what is the best empirical treatment?

A. Penicillin G
B. Doxycycline
C. Streptomycin
D. Vancomycin + ceftriaxone (Correct Answer)

Explanation: **Explanation:** The management of bacterial meningitis is a medical emergency requiring immediate empirical antibiotic therapy before culture results are available [1]. **1. Why Vancomycin + Ceftriaxone is correct:** * **Ceftriaxone (3rd Gen Cephalosporin):** This is the backbone of empirical therapy due to its excellent CSF penetration and broad-spectrum activity against common pathogens like *S. pneumoniae* and *N. meningitidis* [1]. * **Vancomycin:** Due to the rising global prevalence of **Penicillin-resistant *Streptococcus pneumoniae* (PRSP)** and increasing resistance to cephalosporins, Vancomycin is added empirically [3]. This ensures coverage against highly resistant strains until sensitivities are confirmed. * **Synergy:** The combination provides the most reliable bactericidal activity against the most likely causative organisms in an adult. **2. Why other options are incorrect:** * **Penicillin G:** While historically the drug of choice, it is no longer used empirically due to widespread resistance in *S. pneumoniae*. It is only used if the isolate is proven to be highly sensitive [3]. * **Doxycycline:** This is a bacteriostatic drug with poor CSF penetration; it is not indicated for acute bacterial meningitis. * **Streptomycin:** An aminoglycoside that has poor CNS penetration and is primarily used for Tuberculosis or specific infections like Plague/Tularemia, not for acute pneumococcal meningitis. **High-Yield Clinical Pearls for NEET-PG:** * **Dexamethasone:** Should be administered **20 minutes before or with the first dose** of antibiotics to reduce neurological complications (hearing loss/edema) in pneumococcal meningitis [2]. * **Listeria Coverage:** If the patient is >50 years old or immunocompromised, **Ampicillin** must be added to the Vancomycin + Ceftriaxone regimen. * **Drug of Choice (DOC):** Once sensitivity is known, if the strain is penicillin-susceptible, Penicillin G becomes the DOC.

Question 570: A 30-year-old person presents with fever and headache for 20 days. CSF values are: glucose 38 mg/dL, protein 60 mg/dL, and lymphocyte pleocytosis with 20 cells/mm³. What should be included in the initial investigations?

A. Indian ink smear of CSF
B. Acid-Fast Bacilli (AFB) stain
C. Toxoplasmosis testing
D. Herpes Simplex Virus (HSV) detection (Correct Answer)

Explanation: ### Explanation The clinical presentation of a **subacute/chronic headache (20 days)** combined with **lymphocytic pleocytosis**, low-to-normal glucose, and mildly elevated protein points toward a differential diagnosis of chronic meningitis (Tuberculosis, Fungal) or certain viral encephalitides [1]. **Why Option D is Correct:** While HSV typically presents acutely, it is a critical "must-rule-out" diagnosis in any patient presenting with fever, headache, and CSF pleocytosis. In the context of NEET-PG, **HSV-1** is the most common cause of sporadic fatal encephalitis [3]. Even if the duration is slightly longer than the classic acute presentation, HSV PCR is a standard initial investigation because early initiation of Acyclovir significantly improves prognosis. **Analysis of Incorrect Options:** * **B. AFB Stain:** While Tuberculous Meningitis (TBM) is a top differential for chronic meningitis in India, the AFB stain of CSF has extremely low sensitivity (<10–20%). It is rarely included in "initial" investigations as a standalone test; instead, GeneXpert (CBNAAT) or culture is preferred [1]. * **A. India Ink:** This is used for *Cryptococcus neoformans*. While it causes chronic meningitis, it is primarily seen in immunocompromised (HIV+) patients. The cell count here (20 cells/mm³) is low, but without history of immunosuppression, HSV or TB remains more statistically likely. * **C. Toxoplasmosis:** This typically presents as space-occupying lesions (ring-enhancing) in HIV patients rather than a primary meningitis picture with these CSF findings [2]. **NEET-PG High-Yield Pearls:** 1. **CSF in TBM:** Characterized by "Cobweb coagulum," very high protein (>100 mg/dL), and marked hypoglycorrhachia [4]. 2. **HSV Encephalitis:** Shows a predilection for the **temporal lobes** (seen on MRI as hyperintensities) [3]. 3. **Gold Standard for HSV:** CSF PCR is the investigation of choice (Sensitivity >95%). 4. **Lymphocytic Pleocytosis + Low Glucose:** Think TB, Fungal, or occasionally Sarcoidosis/Malignancy. Normal glucose usually points toward Viral causes.

Practice by Chapter

Principles of Antimicrobial Therapy

Practice Questions

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

Practice Questions

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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