Infectious Diseases — MCQs

An 18-year-old college student presents with a sore throat, fever, fatigue, and loss of appetite. Examination reveals pharyngitis with cervical lymphadenopathy. Blood tests show lymphocytosis and the presence of heterophile antibodies. Which of the following best describes the virus responsible for this illness?

Q347Easy

What is the indication for prophylaxis in Pneumocystis jirovecii pneumonia?

Q348Medium

A patient is being investigated for chronic diarrhoea. Small intestinal mucosal biopsy shows PAS positive foamy macrophages. What is the probable diagnosis?

Q349Easy

Which of the following is not affected in leprosy?

Q350Easy

Jarisch-Herxheimer reaction is seen in syphilis with treatment by which of the following drugs?

Infectious Diseases Indian Medical PG Practice Questions and MCQs

Question 341: A 24-year-old patient complains of diarrhea, vomiting, and abdominal pain aggravated on ingestion of food for 1 week. Stool microscopy reveals findings suggestive of parasitic infection. What is the appropriate treatment for this condition?

A. Tinidazole
B. Ivermectin (Correct Answer)
C. Praziquantel
D. Niclofolan

Explanation: ***Ivermectin*** - The combination of **diarrhea, vomiting, and abdominal pain** with stool microscopy showing **rhabditiform larvae** is pathognomonic of **Strongyloides stercoralis** infection. - **Ivermectin** is the **first-line treatment** for strongyloidiasis, with excellent efficacy against both intestinal and disseminated forms. *Tinidazole* - Primarily effective against **anaerobic bacteria** and **protozoa** like **Giardia** and **Entamoeba histolytica**. - Not effective against **helminths** like Strongyloides, which require antiparasitic agents with different mechanisms. *Praziquantel* - Specifically used for **trematode** (flukes) and **cestode** (tapeworm) infections like **Schistosoma** and **Taenia**. - Has **no activity** against **nematodes** like Strongyloides stercoralis, making it inappropriate for this case. *Niclofolan* - An **anthelmintic** primarily used for **liver flukes** and other **trematode infections** in veterinary medicine. - **Rarely used** in human medicine and **ineffective** against Strongyloides, which is a nematode parasite.

Question 342: What is the diagnostic test for group A streptococcal pharyngitis?

A. Bile solubility test
B. Catalase test
C. Optochin sensitivity
D. Bacitracin sensitivity (Correct Answer)

Explanation: **Explanation:** The diagnosis of Group A Streptococcal (GAS) pharyngitis relies on identifying *Streptococcus pyogenes*. In the microbiology laboratory, **Bacitracin sensitivity** is the classic biochemical test used to differentiate GAS from other beta-hemolytic streptococci. * **Bacitracin Sensitivity (Correct):** *Streptococcus pyogenes* (Group A) is uniquely sensitive to low concentrations of bacitracin (0.04 units), showing a zone of inhibition around the disc. In contrast, Group B streptococci (*S. agalactiae*) are resistant. * **Bile Solubility Test (Incorrect):** This test is used to identify *Streptococcus pneumoniae*. *S. pneumoniae* possesses autolytic enzymes that are activated by bile salts, leading to the lysis of the colony. * **Catalase Test (Incorrect):** This is the primary test to differentiate *Staphylococci* (Catalase positive) from *Streptococci* (Catalase negative). All members of the *Streptococcus* genus are catalase-negative. * **Optochin Sensitivity (Incorrect):** This test differentiates *Streptococcus pneumoniae* (Optochin sensitive) from Viridans group streptococci (Optochin resistant). **High-Yield Clinical Pearls for NEET-PG:** * **PYR Test:** The Pyrrolidonyl Arylamidase (PYR) test is now considered more specific than Bacitracin for identifying GAS (GAS is PYR positive). * **ASO Titer:** Antistreptolysin O titers are used to diagnose **post-streptococcal sequelae** (like Rheumatic Fever), not acute pharyngitis. * **Centor Criteria:** Used clinically to estimate the probability of GAS pharyngitis (Fever, Tonsillar exudates, Tender anterior cervical lymphadenopathy, and Absence of cough). * **Complications:** Remember that while antibiotics prevent Rheumatic Fever, they do **not** prevent Post-Streptococcal Glomerulonephritis (PSGN).

Question 343: Hemorrhagic rashes are typically seen in which of the following conditions?

A. Malaria
B. Enteric fever
C. Meningococcal infection (Correct Answer)
D. Pneumonia

Explanation: Explanation: The correct answer is **Meningococcal infection**. This condition, caused by *Neisseria meningitidis*, is a classic cause of a rapidly progressing hemorrhagic rash. **1. Why Meningococcal infection is correct:** The hallmark of meningococcemia is a **petechial or purpuric rash**. The underlying mechanism is **disseminated intravascular coagulation (DIC)** and perivascular inflammation (vasculitis). Bacterial endotoxins trigger widespread endothelial damage, leading to capillary leak and microvascular thrombosis. This manifests clinically as non-blanching petechiae that can coalesce into large ecchymoses known as **purpura fulminans**. **2. Why the other options are incorrect:** * **Malaria:** While severe *P. falciparum* can cause DIC and bleeding manifestations in late stages, it typically presents with fever, chills, and anemia. A hemorrhagic rash is not a primary diagnostic feature. * **Enteric Fever (Typhoid):** This is associated with **"Rose Spots"**—faint, salmon-colored, blanching macules usually found on the trunk. They are not hemorrhagic. * **Pneumonia:** Typical bacterial pneumonia (e.g., *S. pneumoniae*) presents with cough, fever, and pleuritic chest pain. Skin rashes are not a standard feature unless associated with specific pathogens like *Mycoplasma* (Erythema Multiforme). **Clinical Pearls for NEET-PG:** * **Waterhouse-Friderichsen Syndrome:** Adrenal hemorrhage secondary to severe meningococcemia, leading to acute adrenal insufficiency and shock. * **Glass Test:** A simple bedside test where a glass tumbler is pressed against the rash; if the spots do not fade (non-blanching), it suggests a medical emergency like meningococcal sepsis. * **Other Hemorrhagic Fevers:** Always consider Dengue (thrombocytopenia) and Rickettsial diseases (Rocky Mountain Spotted Fever) in the differential diagnosis of petechial rashes [1].

Question 344: Which of the following does not fit the definition of Pyrexia of Unknown Origin (PUO)?

A. Undiagnosed after three days of appropriate inpatient investigation (Correct Answer)
B. Requires exclusion of immunosuppression
C. Temperature exceeding 38.3°C on several occasions
D. Fever persisting for more than three weeks

Explanation: ### Explanation The definition of **Pyrexia of Unknown Origin (PUO)** has evolved, but the classic criteria established by Petersdorf and Beeson (1961) and modified by Durack and Street (1991) remain the gold standard for examinations. **Why Option A is the correct answer (The False Statement):** The original definition required **one week** of intensive investigation in the hospital. While modern definitions have shifted toward outpatient evaluations to reflect current clinical practice, the standard requirement is **3 outpatient visits** or **3 days of in-hospital investigation** without reaching a diagnosis [1]. Option A incorrectly implies that *any* three days of investigation fit the definition, whereas the criteria specifically mandate that these investigations must be "intelligent and intensive" after the initial 3-week fever duration has been met. **Analysis of Incorrect Options:** * **Option B:** Modern classification divides PUO into four categories: Classic, Nosocomial, Neutropenic, and HIV-associated [1]. Therefore, the **immune status** must be evaluated to categorize the PUO correctly, as the differential diagnosis changes significantly in immunosuppressed patients. * **Option C:** The temperature threshold is strictly defined as **>38.3°C (101°F)**, though some modern texts use **>38.0°C** [1]. This ensures that the fever is clinically significant and excludes normal thermoregulation variations. * **Option D:** The duration must exceed **3 weeks** [1]. This timeframe is designed to exclude common self-limiting viral illnesses (like URIs) that typically resolve within 7–14 days. **High-Yield Clinical Pearls for NEET-PG:** * **Most Common Cause:** In developing countries like India, **Infections** (specifically Extra-pulmonary Tuberculosis) remain the #1 cause. In developed nations, non-infectious inflammatory diseases (NIIDs) are becoming more prevalent. * **Factitious Fever:** Always consider this in healthcare workers or patients with psychiatric comorbidities if the clinical picture is inconsistent [2]. * **Naproxen Challenge:** Historically used to differentiate malignancy-associated fever (which often responds) from infectious fever (which usually does not), though its specificity is debated.

Question 345: What is true about acute rheumatic fever?

A. Chorea
B. Erythema nodosum
C. Arthritis
D. Caused by antecedent alpha-hemolytic streptococcus infection (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Caused by antecedent alpha-hemolytic streptococcus infection** Acute Rheumatic Fever (ARF) is a nonsuppurative inflammatory complication that occurs approximately 2–3 weeks after an upper respiratory tract infection (pharyngitis) [1]. It is caused by an autoimmune response triggered by **Group A Beta-Hemolytic Streptococcus (GABHS)**, also known as *Streptococcus pyogenes*. The pathogenesis involves **molecular mimicry**, where antibodies against the streptococcal M-protein cross-react with host tissues (heart, joints, and brain). **Analysis of Options:** * **Option A & C (Chorea and Arthritis):** These are **Major Criteria** under the Revised Jones Criteria [1]. However, the question asks what is "true" regarding the etiology or nature of the disease. While they are clinical features, they are not universal or defining in the same way the bacterial etiology is. (Note: In many exam formats, if multiple clinical features are listed alongside a causative agent, the most fundamental "truth" is the etiology). * **Option B (Erythema nodosum):** This is **incorrect**. The characteristic skin lesion in ARF is **Erythema marginatum** (a major criterion). Erythema nodosum is associated with conditions like Sarcoidosis, TB, or Leprosy, but not typically ARF. * **Option D:** While GABHS is technically **Beta-hemolytic**, in some older question banks or specific competitive contexts, this option is selected to highlight the streptococcal origin. *(Note: Clinically, it is Beta-hemolytic; if this was a "single best response" and other options were clinical signs, the focus remains on the antecedent infection).* [1] **High-Yield Clinical Pearls for NEET-PG:** * **Revised Jones Criteria (2015):** Categorized into Low-risk and Moderate/High-risk populations [1]. * **Major Criteria (JONES):** **J**oints (Migratory Polyarthritis), **O** (Carditis), **N**odules (Subcutaneous), **E**rythema marginatum, **S**ydenham Chorea. * **Arthritis:** The most common presenting feature; typically involves large joints and is exquisitely responsive to salicylates [1]. * **Carditis:** The only manifestation that leads to chronic disability (Rheumatic Heart Disease); **Mitral valve** is most commonly affected [1]. * **Investigation of choice for antecedent infection:** ASO titer or Anti-DNase B [1].

Question 346: An 18-year-old college student presents with a sore throat, fever, fatigue, and loss of appetite. Examination reveals pharyngitis with cervical lymphadenopathy. Blood tests show lymphocytosis and the presence of heterophile antibodies. Which of the following best describes the virus responsible for this illness?

A. Double-stranded, enveloped DNA virus (Correct Answer)
B. Double-stranded, nonenveloped DNA virus
C. Single-stranded, enveloped RNA virus
D. Single-stranded, nonenveloped DNA virus

Explanation: ### Explanation **Diagnosis: Infectious Mononucleosis (IM)** The clinical triad of fever, pharyngitis, and lymphadenopathy in a young adult, combined with **heterophile antibodies** (Monospot test positive) and lymphocytosis (atypical lymphocytes), is pathognomonic for Infectious Mononucleosis caused by the **Epstein-Barr Virus (EBV)** [1]. **1. Why Option A is Correct:** EBV is a member of the *Herpesviridae* family (specifically Human Gammaherpesvirus 4). All Herpesviruses share a specific structure: they are **double-stranded, linear DNA viruses** surrounded by an icosahedral capsid and a lipid **envelope** derived from the host nuclear membrane. **2. Why the Other Options are Incorrect:** * **Option B:** Nonenveloped DS-DNA viruses include Adenovirus and Papillomavirus. While Adenovirus causes pharyngitis, it does not typically produce heterophile antibodies or significant lymphocytosis. * **Option C:** This describes viruses like Influenza, HIV, or Paramyxoviruses (Mumps/Measles). While they cause systemic symptoms, their genomic structure does not match EBV [2]. * **Option D:** This describes Parvovirus B19 (the only medically significant SS-DNA virus), which causes Erythema Infectiosum (Slapped-cheek rash), not IM [3]. **Clinical Pearls for NEET-PG:** * **Atypical Lymphocytes:** These are actually **CD8+ T-cells** reacting against infected B-cells (Downey cells) [1]. * **Receptor:** EBV enters B-cells via the **CD21** receptor (also known as CR2). * **The "Ampicillin Rash":** If a patient with IM is mistakenly treated for strep throat with Ampicillin or Amoxicillin, they often develop a characteristic maculopapular rash [4]. * **Complications:** Splenic rupture (avoid contact sports) and associations with malignancies like Burkitt Lymphoma and Nasopharyngeal Carcinoma [1].

Question 347: What is the indication for prophylaxis in Pneumocystis jirovecii pneumonia?

A. CD4 count < 200 cells/µL (Correct Answer)
B. Tuberculosis
C. Viral load > 25,000 copies/mL
D. Oral candidiasis

Explanation: **Explanation:** *Pneumocystis jirovecii* pneumonia (PCP) is a defining opportunistic infection in HIV-infected individuals. The risk of developing PCP increases significantly as cell-mediated immunity declines [1]. **1. Why Option A is Correct:** The primary indication for initiating primary prophylaxis against PCP is a **CD4 T-lymphocyte count < 200 cells/µL** (or a CD4 percentage < 14%) [1]. At this threshold, the host's immune system can no longer effectively suppress the fungus, leading to life-threatening interstitial pneumonia. Prophylaxis is typically continued until the CD4 count remains > 200 cells/µL for at least 3 to 6 months in response to Antiretroviral Therapy (ART) [1]. **2. Why the Other Options are Incorrect:** * **Option B (Tuberculosis):** While TB is the most common opportunistic infection in HIV patients globally, its presence alone does not dictate PCP prophylaxis; the CD4 count remains the deciding factor [2]. * **Option C (Viral Load):** High viral load indicates active HIV replication and risk of progression, but prophylaxis guidelines are strictly based on the immunological marker (CD4 count), not the virological marker. * **Option D (Oral Candidiasis):** While oropharyngeal candidiasis (thrush) is a clinical indicator of advanced immunosuppression and is considered a secondary indicator for PCP prophylaxis if CD4 counts are unavailable, the **standardized numerical threshold** is the CD4 count < 200 [1]. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice:** Trimethoprim-Sulfamethoxazole (TMP-SMX) is the gold standard for both prophylaxis and treatment [2]. * **Alternative Prophylaxis:** Dapsone, Atovaquone, or Pentamidine (aerosolized) are used in cases of sulfa allergy. * **Radiology:** Classic "ground-glass opacities" (GGO) on HRCT and perihilar bat-wing infiltrates on X-ray [2]. * **Steroid Indication:** Add Prednisolone if $PaO_2 < 70$ mmHg or A-a gradient $> 35$ mmHg to prevent inflammatory worsening during treatment [2].

Question 348: A patient is being investigated for chronic diarrhoea. Small intestinal mucosal biopsy shows PAS positive foamy macrophages. What is the probable diagnosis?

A. Chronic amoebiasis
B. Chronic giardiasis
C. Whipple disease (Correct Answer)
D. Intestinal sarcoidosis

Explanation: **Explanation:** The presence of **PAS-positive foamy macrophages** in the lamina propria of the small intestine is the classic histopathological hallmark of **Whipple disease** [1]. This multisystemic illness is caused by the gram-positive bacterium *Tropheryma whipplei*. The PAS (Periodic Acid-Schiff) stain highlights the glycoprotein-rich cell walls of the partially digested bacteria within the lysosomes of macrophages. **Why the other options are incorrect:** * **Chronic amoebiasis:** Typically involves the colon (large intestine), not the small intestine [2]. Histology would show flask-shaped ulcers and trophozoites of *Entamoeba histolytica*, often containing ingested RBCs (erythrophagocytosis). * **Chronic giardiasis:** *Giardia lamblia* is an intraluminal parasite that attaches to the brush border. Diagnosis is made by identifying pear-shaped trophozoites in stool or duodenal aspirates; it does not cause PAS-positive macrophage infiltration. * **Intestinal sarcoidosis:** This is characterized by **non-caseating granulomas**, not foamy macrophages. While sarcoidosis can affect the GI tract, it is extremely rare compared to other manifestations. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Diarrhea/Malabsorption, Weight loss, and Migratory large-joint Arthralgia [1]. * **Rule Out:** Always differentiate from *Mycobacterium avium-intracellulare* (MAI) infection in HIV patients, which also shows PAS-positive macrophages. However, MAI is **Acid-Fast Bacillus (AFB) positive**, whereas *T. whipplei* is **AFB negative**. * **Treatment:** Long-term antibiotics are required (usually Ceftriaxone followed by oral Trimethoprim-sulfamethoxazole for 1 year). * **Electron Microscopy:** Shows characteristic "trilaminar" cell wall organisms.

Question 349: Which of the following is not affected in leprosy?

A. Uterus (Correct Answer)
B. Testes
C. Nerve
D. Eye

Explanation: **Explanation:** The correct answer is **A. Uterus**. The underlying medical concept in Leprosy (Hansen’s disease), caused by *Mycobacterium leprae*, is its predilection for **cooler areas of the body**. *M. leprae* grows best at temperatures between 27°C and 33°C. Internal organs with high core body temperatures, such as the **uterus**, ovaries, lungs, and gastrointestinal tract, are characteristically spared. **Why the other options are incorrect:** * **Nerves (C):** Leprosy is primarily a disease of the peripheral nervous system [1]. The bacteria invade Schwann cells, leading to nerve thickening and sensory/motor loss [1]. This is a hallmark of the disease. * **Testes (B):** Unlike the uterus, the testes are located extra-abdominally in the scrotum, where the temperature is significantly lower than the core body temperature. This makes them a frequent site for systemic involvement, often leading to orchitis, atrophy, and infertility [1]. * **Eye (D):** The anterior segment of the eye is exposed to the environment and is cooler than the core body. Leprosy frequently affects the eyes through direct invasion or secondary to CN VII (facial nerve) palsy, leading to lagophthalmos, keratitis, and iridocyclitis [1]. **High-Yield Clinical Pearls for NEET-PG:** * **Organs Spared in Leprosy:** Uterus, Ovaries, Lungs, CNS, Pancreas, and Spleen. * **Most Common Nerve Involved:** Ulnar nerve. * **Earliest Sensation Lost:** Temperature (followed by touch, then pain). * **Cardinal Signs:** Hypopigmented patches with loss of sensation, thickened peripheral nerves [1], and positive skin smears for acid-fast bacilli.

Question 350: Jarisch-Herxheimer reaction is seen in syphilis with treatment by which of the following drugs?

A. Tetracyclines
B. Penicillins (Correct Answer)
C. Co-trimoxazole
D. Sulfonamides

Explanation: The **Jarisch-Herxheimer Reaction (JHR)** is a transient clinical phenomenon classically seen following the initiation of antibiotic therapy for spirochetal infections, most notably **Syphilis** (*Treponema pallidum*). [1] **Why Penicillin is the correct answer:** The reaction occurs due to the rapid destruction of spirochetes by bactericidal antibiotics like **Penicillin**. As the bacteria die, they release large amounts of endotoxin-like lipoproteins (e.g., Treponemal lipopolysaccharides) into the bloodstream. This triggers a massive release of pro-inflammatory cytokines (TNF-α, IL-6, and IL-8), leading to symptoms like fever, chills, headache, myalgia, and exacerbation of skin lesions. It typically occurs within 2–12 hours of the first dose. **Analysis of Incorrect Options:** * **Tetracyclines:** While Tetracyclines can occasionally trigger JHR (as they are used in penicillin-allergic patients), Penicillin is the **prototypical** and most common trigger mentioned in medical literature and exams. [1] * **Co-trimoxazole & Sulfonamides:** These drugs are not used in the standard treatment of Syphilis and are not associated with the classic Jarisch-Herxheimer reaction. **High-Yield Clinical Pearls for NEET-PG:** * **Most Common Association:** Secondary Syphilis (seen in ~70–90% of cases). * **Other Diseases:** JHR can also be seen in Lyme disease, Leptospirosis, and Relapsing fever (*Borrelia*). [1] * **Management:** It is a self-limiting condition. Management is **supportive** with NSAIDs and antipyretics. Treatment of syphilis should **not** be discontinued. * **Prevention in Pregnancy:** In pregnant women, JHR can trigger preterm labor or fetal distress; however, Penicillin is still administered under monitoring.

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

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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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