Infectious Diseases — MCQs

Question 1: A 28-year-old woman presented with high-grade fever, cough, diarrhea, and mental confusion for 4 days. Chest X-ray revealed bilateral pneumonitis. What etiology is most likely?

• A. Streptococcus pneumoniae
• B. Staphylococcus aureus
• C. Legionella pneumophila (Correct Answer)
• D. Pseudomonas aeruginosa

Explanation: ### Explanation The clinical presentation of **Legionella pneumophila** (Legionnaires' disease) is classically characterized by a "pneumonia plus" syndrome—respiratory symptoms accompanied by prominent multisystem involvement. **Why Legionella is correct:** 1. **Multisystem Involvement:** Unlike typical bacterial pneumonias, Legionella frequently presents with **extrapulmonary symptoms**. The combination of **high-grade fever**, **diarrhea** (gastrointestinal), and **mental confusion** (neurological) in a patient with pneumonitis is a classic "textbook" description of Legionellosis [1]. 2. **Radiology:** It often presents with bilateral patchy infiltrates or consolidation that may appear worse than the clinical exam suggests. 3. **Laboratory Clue (High-Yield):** Though not mentioned in the stem, **hyponatremia** is a highly specific laboratory finding associated with this pathogen. **Why other options are incorrect:** * **Streptococcus pneumoniae:** The most common cause of community-acquired pneumonia (CAP) [1]. It typically presents with sudden onset chills, productive cough (rusty sputum), and lobar consolidation, but lacks the prominent GI and CNS symptoms seen here. * **Staphylococcus aureus:** Usually follows a viral prodrome (like Influenza) and often leads to necrotizing pneumonia with cavitations or pneumatoceles, rather than multisystem involvement [1]. * **Pseudomonas aeruginosa:** Primarily seen in immunocompromised patients, those with structural lung disease (Cystic Fibrosis, Bronchiectasis), or hospital-acquired settings. It does not typically cause diarrhea or confusion. **NEET-PG Clinical Pearls:** * **Diagnosis:** The most rapid test is the **Urinary Antigen Test** (detects Serogroup 1). The gold standard is culture on **Buffered Charcoal Yeast Extract (BCYE) agar**. * **Treatment:** Macrolides (Azithromycin) or Fluoroquinolones (Levofloxacin). It is inherently resistant to Beta-lactams because it is an intracellular pathogen. * **Transmission:** Associated with contaminated water systems, cooling towers, and air conditioners (no person-to-person spread) [1].

Question 2: A rash appeared 9 hours after a scuba dive. What is the diagnosis?

• A. Decompression sickness (Correct Answer)
• B. Jellyfish envenomation
• C. Mycobacterium marinum infection
• D. Phylum Porifera contact dermatitis

Question 3: Which of the following statements regarding Pneumocystis jirovecii pneumonia (PCP) is/are true? 1. Bronchoscopy with bronchoalveolar lavage (BAL) is the mainstay of diagnosis for PCP. 2. Pneumatoceles are seen in all cases of PCP. 3. A CD4 count < 350 cells/µL is an indication for prophylaxis against PCP. 4. Trimethoprim-sulfamethoxazole is the drug of choice for prophylaxis.

• A. 1 - True, 2 - False, 3 - False, 4 - True (Correct Answer)
• B. 1 - False, 2 - True, 3 - True, 4 - False
• C. 1 - True, 2 - True, 3 - False, 4 - True
• D. 1 - False, 2 - False, 3 - True, 4 - False

Explanation: **Explanation:** **1. Bronchoscopy with BAL is the mainstay of diagnosis (True):** *Pneumocystis jirovecii* cannot be cultured. Diagnosis relies on microscopic visualization of cysts or trophic forms in respiratory specimens [1]. While induced sputum is the initial non-invasive step, **Bronchoalveolar Lavage (BAL)** is the gold standard diagnostic procedure due to its high sensitivity (>90-95%) [1]. **2. Pneumatoceles are seen in all cases (False):** The classic radiological finding in PCP is **bilateral diffuse ground-glass opacities** (perihilar distribution). While pneumatoceles (thin-walled air-filled cysts) can occur and increase the risk of spontaneous pneumothorax, they are seen in only about 10-20% of cases, not all [1]. **3. CD4 count < 350 cells/µL is the indication for prophylaxis (False):** In HIV-infected patients, primary prophylaxis for PCP is indicated when the **CD4 count falls below 200 cells/µL** (or if there is a history of oropharyngeal candidiasis) [1]. A threshold of 350 cells/µL is too high for initiating PCP-specific prophylaxis. **4. Trimethoprim-sulfamethoxazole (TMP-SMX) is the drug of choice (True):** TMP-SMX is the first-line agent for both the **treatment and prophylaxis** of PCP [1]. It is highly effective and also provides cross-protection against Toxoplasmosis. **High-Yield Clinical Pearls for NEET-PG:** * **Stains:** Use **Gomori Methenamine Silver (GMS)** to see crushed-ping-pong-ball shaped cysts or **Giemsa** for trophic forms [1]. * **Biomarker:** Elevated **Serum Beta-D-Glucan** has high sensitivity (negative predictive value) for PCP [2]. * **Steroids:** Indicated in treatment if **PaO2 < 70 mmHg** or Alveolar-arterial (A-a) gradient > 35 mmHg to prevent inflammatory worsening [1]. * **Alternative for Prophylaxis:** Dapsone or Atovaquone (if sulfa-allergic) [1].

Question 4: What is the most likely diagnosis for this 19-month-old girl who presented with a 1-week history of an ascending erythematous eruption?

• A. Cellulitis
• B. Hand, foot, and mouth disease
• C. Herpes zoster (Correct Answer)
• D. Lymphadenitis

Explanation: **Explanation:** The correct diagnosis is **Herpes Zoster (Shingles)**. While commonly associated with the elderly, Herpes Zoster can occur in children, especially those who were infected with Varicella Zoster Virus (VZV) in utero or during the first year of life. The "ascending erythematous eruption" in this clinical scenario refers to a **dermatomal distribution**. Herpes Zoster is caused by the reactivation of latent VZV from the dorsal root ganglia. In pediatric patients, the presentation is often milder than in adults, frequently lacking the classic prodromal pain or post-herpetic neuralgia, which can make the diagnosis challenging if the clinician only expects "adult-type" symptoms. **Why the other options are incorrect:** * **Cellulitis:** This typically presents as a spreading, poorly demarcated area of skin erythema, warmth, and edema. It does not follow a dermatomal pattern and is usually associated with systemic signs like high fever. * **Hand, Foot, and Mouth Disease (HFMD):** Caused by Coxsackievirus A16, this presents with vesicular lesions on the palms, soles, and oral mucosa. It is not dermatomal or "ascending" in a linear nerve distribution. * **Lymphadenitis:** This refers to the inflammation/infection of lymph nodes. While it may cause localized swelling and redness, it does not present as a vesicular or erythematous eruption following a nerve path. **NEET-PG High-Yield Pearls:** * **Risk Factor:** The single greatest risk factor for pediatric Herpes Zoster is **primary varicella infection before age 1**. * **Distribution:** The most common sites in children are the **thoracic** and **lumbar** dermatomes. * **Diagnosis:** Usually clinical; however, the **Tzanck smear** (showing multinucleated giant cells) or PCR are definitive. * **Management:** In healthy children, the condition is often self-limiting; acyclovir is reserved for immunocompromised patients or severe cases.

Question 5: Fever increases water losses by ______ ml/day per degree Celsius?

• A. 100
• B. 200 (Correct Answer)
• C. 400
• D. 800

Explanation: **Explanation:** The correct answer is **200 ml/day per degree Celsius**. **1. Underlying Medical Concept:** In healthy adults, insensible water loss (via skin and lungs) typically accounts for approximately 600–900 ml/day [2]. When a patient develops a fever, the metabolic rate increases (roughly 10–13% for every 1°C rise) [1], leading to increased respiratory rate (tachypnea) and cutaneous vasodilation/diaphoresis. This physiological response significantly accelerates insensible fluid loss. Standard clinical teaching and medical textbooks (such as Harrison’s Principles of Internal Medicine) state that for every degree Celsius rise in body temperature above normal, daily fluid requirements increase by approximately **200 ml/day**. **2. Analysis of Incorrect Options:** * **Option A (100 ml):** This underestimates the physiological demand. While 100 ml might be lost in very mild febrile states, it does not represent the standard clinical calculation used for fluid resuscitation in high-grade fever. * **Option C (400 ml) & D (800 ml):** These values are too high for a single degree Celsius increase. Such volumes are more characteristic of total losses in patients with severe tachypnea, extensive burns, or prolonged high-grade fevers (e.g., 39–40°C) rather than the incremental loss per degree. **3. High-Yield Clinical Pearls for NEET-PG:** * **Metabolic Rate:** For every 1°C rise in temperature, the Basal Metabolic Rate (BMR) increases by approximately **13%** [1]. * **Pediatric Variation:** In children, the rule is often simplified to an additional **10–12%** of maintenance fluids for each degree above 38°C. * **Insensible Loss Components:** Remember that roughly 70% of insensible loss occurs through the skin and 30% through the respiratory tract [2]. Both are exacerbated by fever. * **Clinical Application:** When calculating "Maintenance Fluid" in a febrile patient, always add the incremental loss (200 ml/°C) to the baseline requirement to prevent dehydration.