Infectious Diseases — MCQs

Over a holiday weekend, more than 100 adults at a resort hotel develop a diarrheal illness marked by voluminous, watery stools more than 10 times per day. They also report headaches, abdominal cramping pain, and myalgias. On physical examination, they have manifestations of dehydration and mild fever. Laboratory studies of stool samples show no increase in leukocytes or fat and no RBCs. Their illness lasts just 1 to 3 days and resolves with no sequelae. Which of the following infectious agents is the most likely cause of their illness?

Q538Medium

A 70-year-old nursing home patient developed influenza, refused the influenza vaccine, and died of an acute pneumonic attack one week after the initial influenza infection. What is the most common cause of acute post-influenza pneumonia?

Q539Medium

Which of the following is FALSE regarding lymphoma in patients with Acquired Immunodeficiency Syndrome (AIDS)?

Q540Medium

Which of the following is NOT a treatment for neurocysticercosis?

Infectious Diseases Indian Medical PG Practice Questions and MCQs

Question 531: A patient presented with fever for 8 days and difficulty in breathing for two days. On examination, a red painless rash was found on the left side of the chest. What is the appropriate treatment?

A. Oseltamivir
B. Streptomycin
C. Doxycycline (Correct Answer)
D. Ceftriaxone

Explanation: ### Explanation The clinical presentation of prolonged fever (8 days), respiratory distress, and a **red painless rash** (specifically an **eschar**) is highly suggestive of **Scrub Typhus**, a rickettsial infection caused by *Orientia tsutsugamushi* [1]. **1. Why Doxycycline is Correct:** The "red painless rash" described is the classic **Eschar**—the hallmark of Scrub Typhus. It represents the site of a larval mite (chigger) bite, characterized by a necrotic center with a surrounding erythematous halo [1]. Scrub Typhus often presents with complications like **ARDS (Acute Respiratory Distress Syndrome)**, explaining the patient's difficulty in breathing. **Doxycycline** is the drug of choice for rickettsial diseases, acting by inhibiting bacterial protein synthesis. **2. Why Other Options are Incorrect:** * **Oseltamivir:** Used for Influenza. While flu causes fever and respiratory symptoms, it does not present with an eschar. * **Streptomycin:** An aminoglycoside used for Tuberculosis or Plague. It is not effective against intracellular rickettsial organisms. * **Ceftriaxone:** A third-generation cephalosporin used for bacterial pneumonia or meningitis. Rickettsial organisms are inherently resistant to beta-lactams. **3. NEET-PG High-Yield Pearls:** * **Vector:** Larval stage of Trombiculid mite (**Chigger**) [1]. * **Diagnosis:** **Weil-Felix test** (Heterophile antibody test) is a classic screening tool (OX-K positive), but **IgM ELISA** is now the gold standard. * **Common Complications:** ARDS, Meningoencephalitis, and Multi-Organ Dysfunction Syndrome (MODS). * **Alternative Treatment:** Azithromycin is preferred in pregnancy or for doxycycline-resistant strains [1].

Question 532: Which of the following pulmonary pathogens are associated with long-term glucocorticoid therapy? 1. M. tuberculosis 2. P. aeruginosa 3. S. aureus 4. Nocardia sp.

A. 3 only
B. 1 and 4
C. 2 and 3
D. 2, 3 and 4 (Correct Answer)

Explanation: ### Explanation The use of long-term glucocorticoids (typically defined as ≥20 mg of prednisone daily for >2 weeks) significantly impairs cell-mediated immunity, neutrophil function, and macrophage activation [1]. This predisposes patients to a specific spectrum of opportunistic and pyogenic pulmonary infections. **Why Option D (2, 3, and 4) is Correct:** * **S. aureus and P. aeruginosa:** Glucocorticoids inhibit neutrophil chemotaxis and phagocytosis. This defect in the innate immune response increases the risk of severe infections by pyogenic bacteria, particularly *Staphylococcus aureus* and Gram-negative bacilli like *Pseudomonas aeruginosa*. * **Nocardia sp.:** *Nocardia* is a classic opportunistic pathogen in steroid-treated patients. Steroids impair the oxidative burst of macrophages, which is essential for killing these filamentous bacteria. **Why Option 1 (M. tuberculosis) is excluded in this specific context:** While steroids *do* increase the risk of reactivating *M. tuberculosis*, the question asks for pathogens specifically associated with the immunosuppression profile of long-term therapy in a comparative context. In many standard medical curricula (and Harrison’s Principles of Internal Medicine), *S. aureus*, *P. aeruginosa*, and *Nocardia* are grouped together as the primary bacterial/higher bacterial threats, whereas *M. tuberculosis* is often categorized separately under chronic granulomatous risks or latent reactivation. In the context of this specific MCQ, the focus is on the triad of pyogenic and higher bacterial opportunistic infections. **High-Yield Clinical Pearls for NEET-PG:** * **Steroids & Fungi:** Long-term steroid use is the strongest risk factor for **Invasive Aspergillosis** and *Pneumocystis jirovecii* pneumonia (PCP). * **Dose Threshold:** A dose of **>20 mg/day of Prednisone** is generally considered the threshold for significant immunosuppression [1]. * **Nocardia vs. Actinomyces:** Remember that *Nocardia* is aerobic and acid-fast (associated with steroids), while *Actinomyces* is anaerobic and non-acid-fast (associated with poor oral hygiene).

Question 533: As per the Revised National Tuberculosis Control Programme (RNTCP) 2010 guidelines, a tuberculosis patient with only rifampicin resistance will be treated under which category?

A. Category I
B. Category II
C. Category III
D. Category IV (Correct Answer)

Explanation: ### Explanation **1. Why Category IV is Correct:** Under the **RNTCP 2010 guidelines**, patients diagnosed with **Drug-Resistant Tuberculosis (DR-TB)**—specifically those with Rifampicin resistance (RR-TB) or Multidrug-resistant TB (MDR-TB, defined as resistance to at least Isoniazid and Rifampicin)—were managed under **Category IV**. This category utilized a standardized regimen of second-line drugs (SLDs) for a duration of 24–27 months. The rationale was that Rifampicin is the most potent bactericidal drug; resistance to it necessitates a specialized, longer, and more toxic regimen to ensure cure and prevent further transmission. **2. Why Other Options are Incorrect:** * **Category I:** Reserved for **newly diagnosed** smear-positive or seriously ill smear-negative pulmonary and extra-pulmonary TB cases. These patients were treated with the standard 6-month HRZE regimen [1]. * **Category II:** Reserved for **previously treated cases** (Relapse, Treatment after Failure, or Treatment after Loss to Follow-up). It involved an 8-month regimen including Streptomycin. It did not cover drug-resistant cases. * **Category III:** Historically used for smear-negative or extra-pulmonary TB cases that were not seriously ill. However, by 2010, Category III was largely phased out and merged into Category I. **3. NEET-PG High-Yield Pearls:** * **Evolution of Guidelines:** RNTCP has been renamed the **National TB Elimination Programme (NTEP)**. Current guidelines have moved away from "Categories" toward **Universal Drug Susceptibility Testing (UDST)**. * **MDR-TB Definition:** Resistance to both Isoniazid (H) and Rifampicin (R). * **XDR-TB Definition (Updated):** MDR-TB plus resistance to any fluoroquinolone AND at least one additional Group A drug (Bedaquiline or Linezolid). * **Diagnosis:** CBNAAT (GeneXpert) is the preferred initial tool as it detects both *M. tuberculosis* and Rifampicin resistance simultaneously within 2 hours.

Question 534: Which of the following parasites can cause duodenal stricture?

A. Ascaris lumbricoides
B. Strongyloides stercoralis (Correct Answer)
C. Chlonorchis sinensis
D. Ankylostoma duodenale

Explanation: **Explanation:** **Strongyloides stercoralis** is the correct answer because of its unique ability to cause chronic, transmural inflammation in the upper gastrointestinal tract. Unlike most helminths, *Strongyloides* can undergo an **autoinfection cycle**, leading to a high worm burden (hyperinfection) [1]. The larvae penetrate the duodenal and jejunal mucosa, triggering an intense inflammatory response, eosinophilic infiltration, and subsequent fibrosis. Over time, this chronic inflammation leads to **cicatrization and duodenal strictures**, which may present clinically as gastric outlet obstruction. **Analysis of Incorrect Options:** * **Ascaris lumbricoides:** While it is the most common cause of intestinal obstruction (due to a physical bolus of worms), it typically affects the ileum and causes **mechanical** rather than structural/fibrotic strictures. * **Clonorchis sinensis:** This is a liver fluke. It primarily inhabits the **biliary tree**, leading to cholangitis, biliary strictures, and cholangiocarcinoma, but it does not cause duodenal strictures. * **Ankylostoma duodenale:** Hookworms attach to the duodenal mucosa to suck blood, leading to **iron deficiency anemia**. They do not invade the wall deeply enough to cause transmural fibrosis or stricture formation [2]. **NEET-PG Clinical Pearls:** * **Drug of Choice:** Ivermectin is the preferred treatment for Strongyloidiasis (Albendazole is an alternative) [1]. * **Hyperinfection Syndrome:** Often triggered by **corticosteroid therapy**, which enhances the transformation of rhabditiform larvae into invasive filariform larvae [1]. * **Diagnosis:** Look for **rhabditiform larvae** in the stool (not eggs) or use the **Entero-test (String test)** [1]. * **Larva Currens:** A pathognomonic, rapidly moving serpiginous cutaneous eruption associated with *Strongyloides* [1].

Question 535: Painless oral ulcers are associated with which of the following conditions?

A. Secondary herpes
B. Primary syphilis (Correct Answer)
C. Tuberculosis
D. Primary herpes

Explanation: ### Explanation **Correct Answer: B. Primary syphilis** **1. Why Primary Syphilis is Correct:** Primary syphilis is characterized by the **chancre**, which typically appears at the site of inoculation (including the lips, tongue, or oral mucosa). The hallmark of a syphilitic chancre is that it is a **painless**, indurated, and clean-based ulcer. It is often accompanied by painless regional lymphadenopathy. This lack of pain is a critical diagnostic differentiator in clinical practice and exams. **2. Why Other Options are Incorrect:** * **A & D (Secondary and Primary Herpes):** Infections caused by Herpes Simplex Virus (HSV-1 or HSV-2) are characteristically **painful**. Primary herpes often presents as acute gingivostomatitis with multiple painful vesicles and ulcers, while secondary (recurrent) herpes presents as painful "cold sores" or fever blisters. * **C (Tuberculosis):** Oral TB ulcers are rare but typically **very painful**. they often present as irregular, ragged ulcers on the dorsum of the tongue, usually secondary to pulmonary TB. **3. NEET-PG High-Yield Clinical Pearls:** * **Syphilis Rule of Thumb:** Primary (Chancre) and Secondary (Condyloma lata) lesions are generally **painless** but highly infectious. * **Painful vs. Painless Ulcers (Genital/Oral):** * **Painless:** Syphilis, Lymphogranuloma venereum (initial lesion), Donovanosis (Granuloma inguinale - usually painless but beefy red). * **Painful:** Herpes (HSV), Chancroid (*Haemophilus ducreyi* — "You *do cry* because it's painful"), Behçet’s disease, and Aphthous stomatitis. * **Diagnosis:** The gold standard for primary syphilis is **Darkfield Microscopy** (showing motile *Treponema pallidum*), as serological tests (VDRL/RPR) may still be negative in the early stages.

Question 536: A patient with cholera experiencing diarrhea presents with a weak thready pulse, clammy skin, hypotension, and central nervous system symptoms. This clinical presentation typically occurs when the fluid loss is approximately what percentage of body weight?

A. 2%
B. 5%
C. 7%
D. >10% (Correct Answer)

Explanation: ### Explanation In Cholera, the severity of dehydration is directly proportional to the volume of fluid lost, which is categorized into mild, moderate, and severe stages based on the percentage of body weight lost. **1. Why >10% is Correct:** Loss of **>10% of body weight** indicates **Severe Dehydration**. At this stage, the intravascular volume is critically depleted, leading to **hypovolemic shock**. Clinical hallmarks include: * **Hemodynamic instability:** Hypotension and a weak, thready, or absent radial pulse [1]. * **Peripheral vasoconstriction:** Cold, clammy skin and cyanosis. * **Neurological impairment:** Altered sensorium, lethargy, or coma due to decreased cerebral perfusion [1]. * **Other signs:** Anuria and deeply sunken eyes. **2. Why other options are incorrect:** * **2% (Mild Dehydration):** Fluid loss is minimal. The patient is thirsty but hemodynamically stable with moist mucous membranes. * **5% (Mild to Moderate Dehydration):** Characterized by increased thirst, dry mouth, and slightly decreased skin turgor, but blood pressure remains normal. * **7% (Moderate Dehydration):** Presents with a rapid pulse, loss of skin elasticity (skin pinch goes back slowly), and irritability. However, the patient is not yet in frank shock or showing CNS depression. ### NEET-PG High-Yield Pearls: * **Gold Standard Treatment:** For severe dehydration (>10%), the immediate treatment is **aggressive IV resuscitation** with **Ringer’s Lactate** (preferred over Normal Saline as it better addresses metabolic acidosis). * **Drug of Choice:** While rehydration is primary, **Azithromycin** is currently the preferred antibiotic for cholera (single dose), though Doxycycline is also used. * **Stool Characteristics:** "Rice-water stools" with a fishy odor; non-bloody and non-mucoid [1]. * **Electrolyte Profile:** Hypokalemia and Hyperchloremic metabolic acidosis are common.

Question 537: Over a holiday weekend, more than 100 adults at a resort hotel develop a diarrheal illness marked by voluminous, watery stools more than 10 times per day. They also report headaches, abdominal cramping pain, and myalgias. On physical examination, they have manifestations of dehydration and mild fever. Laboratory studies of stool samples show no increase in leukocytes or fat and no RBCs. Their illness lasts just 1 to 3 days and resolves with no sequelae. Which of the following infectious agents is the most likely cause of their illness?

A. Cytomegalovirus
B. Clostridium botulinum
C. Norovirus (Correct Answer)
D. Staphylococcus aureus

Explanation: ### Explanation The clinical presentation describes a classic outbreak of **Norovirus** gastroenteritis, the most common cause of viral gastroenteritis outbreaks worldwide, often associated with closed or semi-closed communities like resorts, cruise ships, and schools. **1. Why Norovirus is Correct:** * **Epidemiology:** Large-scale outbreaks over a short period (holiday weekend) suggest a common-source viral pathogen. * **Clinical Features:** Characterized by sudden onset of voluminous watery diarrhea, vomiting, and systemic symptoms like headache, myalgias, and low-grade fever. * **Stool Analysis:** The absence of fecal leukocytes, RBCs, or fat indicates a **non-inflammatory, secretory process** typical of viral infections that affect the small intestine without causing mucosal invasion or malabsorption. * **Duration:** The self-limiting nature (1–3 days) is a hallmark of Norovirus [1]. **2. Why Other Options are Incorrect:** * **A. Cytomegalovirus (CMV):** Typically causes diarrhea in immunocompromised patients (e.g., HIV/AIDS). It presents with chronic, bloody diarrhea and biopsy would show characteristic "owl’s eye" inclusions. * **B. Clostridium botulinum:** Causes descending paralysis and autonomic dysfunction due to neurotoxin ingestion. While it can occur in outbreaks (canned food), it does not present with voluminous watery diarrhea or fever. * **C. Staphylococcus aureus:** Causes food poisoning via preformed enterotoxins. While it causes outbreaks, the incubation period is very short (1–6 hours), and **vomiting** is the predominant symptom rather than prolonged watery diarrhea [2]. **3. NEET-PG High-Yield Pearls:** * **Kaplan Criteria:** Used to diagnose Norovirus outbreaks (Vomiting in >50% of cases, mean incubation 24–48h, mean duration 12–60h, stool cultures negative for bacteria). * **Transmission:** Fecal-oral route; highly contagious due to low infectious dose and resistance to common disinfectants (alcohol-based sanitizers are often ineffective; bleach is preferred). * **Mechanism:** Blunting of villi in the proximal small intestine leads to transient malabsorption and decreased brush border enzyme activity.

Question 538: A 70-year-old nursing home patient developed influenza, refused the influenza vaccine, and died of an acute pneumonic attack one week after the initial influenza infection. What is the most common cause of acute post-influenza pneumonia?

A. Measles
B. Staphylococcus aureus (Correct Answer)
C. Cytomegalovirus
D. Legionella

Explanation: **Explanation:** The clinical scenario describes **Secondary Bacterial Pneumonia**, a classic complication of influenza, particularly in elderly or immunocompromised patients [1]. Influenza virus damages the respiratory epithelium and impairs ciliary clearance, creating a fertile environment for bacterial superinfection. **Why Staphylococcus aureus is correct:** While *Streptococcus pneumoniae* remains the most common cause of community-acquired pneumonia overall, **Staphylococcus aureus** (including MRSA) is the most characteristic and feared pathogen specifically following an influenza infection [1]. It often presents as a severe, necrotizing pneumonia with a high mortality rate, occurring roughly 1–2 weeks after the initial viral symptoms seem to be resolving (the "biphasic" presentation). **Analysis of Incorrect Options:** * **A. Measles:** This is a viral infection itself, not a secondary bacterial complication of influenza. While measles can cause pneumonia (Hecht’s giant cell pneumonia), it is unrelated to post-influenza sequelae. * **C. Cytomegalovirus (CMV):** CMV pneumonia typically occurs in severely immunocompromised patients (e.g., post-transplant or HIV/AIDS). It is not a standard complication of seasonal influenza. * **D. Legionella:** This causes "Atypical Pneumonia" often associated with contaminated water systems or air conditioning [1]. It does not have a specific pathophysiological link to post-influenza states. **NEET-PG High-Yield Pearls:** * **Most common cause of post-viral pneumonia:** *Streptococcus pneumoniae* (highest incidence). * **Most "characteristic" or "specific" post-viral pneumonia:** *Staphylococcus aureus* (often tested due to its severity and association with cavitary lesions/pneumatoceles) [1]. * **Pathogenesis:** Viral neuraminidase cleaves sialic acid, exposing receptors for bacterial attachment. * **Clinical Clue:** A patient who gets better from the flu and then suddenly develops high fever, productive cough, and new infiltrates.

Question 539: Which of the following is FALSE regarding lymphoma in patients with Acquired Immunodeficiency Syndrome (AIDS)?

A. Intranodal involvement
B. Extranodal involvement
C. Highly aggressive nature
D. Less responsive to treatment (Correct Answer)

Explanation: The landscape of HIV-associated lymphomas has evolved significantly with the advent of **Combined Antiretroviral Therapy (cART)**. **Why Option D is the Correct (False) Statement:** Historically, AIDS-related lymphomas (ARL) had a dismal prognosis. However, in the modern cART era, the statement that they are "less responsive to treatment" is no longer accurate. When treated with standard chemo-immunotherapy (like R-CHOP or dose-adjusted EPOCH-R) alongside cART, HIV-positive patients achieve **remission rates and overall survival outcomes comparable to HIV-negative patients**. Therefore, HIV status alone is no longer a primary predictor of poor treatment response. **Analysis of Incorrect Options:** * **Option A & B:** ARL is characterized by its atypical distribution. While it can involve lymph nodes (**Intranodal**), a hallmark of AIDS-related B-cell lymphomas is the high frequency of **Extranodal involvement** (seen in up to 80% of cases), commonly affecting the CNS, GI tract, liver, and bone marrow [1]. * **Option C:** These lymphomas are characteristically **Highly Aggressive** [2]. The most common subtypes are Diffuse Large B-cell Lymphoma (DLBCL) and Burkitt Lymphoma, both of which exhibit rapid clinical progression and high-grade histological features [2]. **High-Yield Clinical Pearls for NEET-PG:** * **Primary CNS Lymphoma:** Strongly associated with **EBV infection** (detected via CSF PCR) and typically occurs at CD4 counts **<50 cells/µL** [3]. * **Burkitt Lymphoma:** Often occurs at relatively higher CD4 counts (**>200 cells/µL**) compared to other ARLs. * **HHV-8:** Associated with Primary Effusion Lymphoma (PEL) and Kaposi Sarcoma. * **Key Management:** The most critical step in improving survival is the **concomitant administration of cART** with chemotherapy.

Question 540: Which of the following is NOT a treatment for neurocysticercosis?

A. Albendazole
B. Ivermectin (Correct Answer)
C. Praziquantel
D. Surgery

Explanation: Neurocysticercosis (NCC), caused by the larval stage of *Taenia solium*, requires a multi-modal treatment approach depending on the stage and location of the cysts [1]. **Why Ivermectin is the correct answer:** Ivermectin is a broad-spectrum antiparasitic agent primarily used for Strongyloidiasis, Onchocerciasis, and Scabies [4]. It does **not** have a proven clinical role in the treatment of neurocysticercosis. The standard cysticidal drugs are Albendazole and Praziquantel [2]. **Analysis of other options:** * **Albendazole (Option A):** The drug of choice for NCC. It has better CNS penetration than Praziquantel and is more effective at killing viable cysts [2]. * **Praziquantel (Option B):** An alternative or adjunct cysticidal agent [1]. In cases of multiple parenchymal cysts, combination therapy (Albendazole + Praziquantel) is now often recommended to increase the rate of cyst clearance. * **Surgery (Option D):** Indicated in specific complications such as intraventricular cysts (endoscopic removal), hydrocephalus (VP shunt), or subarachnostroid NCC with mass effect. **High-Yield Clinical Pearls for NEET-PG:** 1. **Steroids First:** Always administer corticosteroids (e.g., Dexamethasone) *before* or concurrently with cysticidal drugs to prevent neurological worsening due to the inflammatory response to dying larvae. 2. **Seizure Control:** Anti-epileptic drugs (AEDs) are the first line of management for patients presenting with seizures [3]. 3. **Contraindication:** Cysticidal drugs are generally **avoided** in Cysticercal Encephalitis (diffuse brain edema) and Ocular Cysticercosis (risk of irreversible blindness due to inflammation). 4. **Calcified Cysts:** These are dead larvae; they do not require antiparasitic treatment, only symptomatic management for seizures.

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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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