Infectious Diseases — MCQs
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A 22-year-old man consumed a beef sandwich and potato salad, experiencing vomiting 2 hours after the meal. What is the probable diagnosis?
A 25-year-old male presented with high-grade fever, headache, and neck stiffness. On examination, neck rigidity and positive Kernig's sign were found. CSF analysis showed neutrophilic predominance, low glucose, and a positive Limulus Amebocyte Lysate assay. Which of the following is the likely pathogen?
Pancytopenia is most common after which condition?
Primary complex in which of the following sites suggests congenital tuberculosis?
What is the antibiotic of choice for the prophylaxis of endocarditis in adults undergoing dental procedures, considering penicillin allergy and inability to take oral medication?
A 33-year-old man has never been vaccinated for hepatitis B. Serologic tests reveal negative hepatitis B surface antigen (HBsAg) and positive antibody to surface antigen. Which of the following conditions does this serologic pattern best fit with?
A 47-year-old man suddenly develops high fever and hypotension. He has a generalized erythematous macular rash, and over the next day, develops gangrene of his left leg. Which of the following is the most likely organism?
Prophylaxis for Pneumocystis jirovecii pneumonia is indicated in HIV-positive patients when the CD4 count is:
Fleeting skin lesions are often present in patients with:
Actinomycosis is commonly seen in which of the following bones?
Infectious Diseases Indian Medical PG Practice Questions and MCQs
Question 691: A 22-year-old man consumed a beef sandwich and potato salad, experiencing vomiting 2 hours after the meal. What is the probable diagnosis?
- A. Staphylococcus aureus poisoning (Correct Answer)
- B. Bacillus cereus poisoning
- C. Clostridial poisoning
- D. Helicobacter pylori poisoning
Explanation: **Explanation:** The clinical presentation of rapid-onset vomiting (within 1–6 hours) after consuming contaminated food is characteristic of **Staphylococcus aureus** food poisoning. **1. Why Staphylococcus aureus is correct:** The key to this diagnosis is the **incubation period**. *S. aureus* produces a heat-stable enterotoxin in food (like potato salad or processed meats) before it is consumed. Because the toxin is pre-formed, symptoms appear very quickly—typically within **1 to 6 hours**. Vomiting is the predominant symptom due to the toxin's effect on the vagus nerve and the vomiting center in the brain. **2. Why other options are incorrect:** * **Bacillus cereus:** While the emetic form of *B. cereus* also has a short incubation (1–5 hours), it is classically associated with **reheated fried rice**. While possible, *S. aureus* is the more classic association for mayonnaise-based salads (potato salad) and meats. * **Clostridial poisoning:** *Clostridium perfringens* has a longer incubation period (**8–16 hours**) and primarily causes watery diarrhea and abdominal cramps, with vomiting being rare. * **Helicobacter pylori:** This organism causes chronic gastritis and peptic ulcers; it does not cause acute, short-incubation food poisoning. **NEET-PG High-Yield Pearls:** * **Shortest Incubation (1–6 hrs):** Think *S. aureus* (meats/salads) or *B. cereus* (rice). * **Intermediate Incubation (8–16 hrs):** Think *C. perfringens*. * **Long Incubation (>16 hrs):** Think *Vibrio cholerae*, *Salmonella*, or *E. coli* (requires organism growth in the gut). * **S. aureus toxin:** It is **heat-stable** (resists boiling for 30 minutes), meaning reheating food does not prevent the illness.
Question 692: A 25-year-old male presented with high-grade fever, headache, and neck stiffness. On examination, neck rigidity and positive Kernig's sign were found. CSF analysis showed neutrophilic predominance, low glucose, and a positive Limulus Amebocyte Lysate assay. Which of the following is the likely pathogen?
- A. Staphylococcus aureus
- B. Streptococcus pneumoniae
- C. Neisseria meningitidis (Correct Answer)
- D. Listeria monocytogenes
Explanation: **Explanation:** The clinical presentation of high-grade fever, headache, neck stiffness, and a positive Kernig’s sign is classic for **Acute Bacterial Meningitis** [1]. The CSF analysis showing neutrophilic pleocytosis and low glucose further confirms a bacterial etiology [1]. The definitive clue in this question is the **positive Limulus Amebocyte Lysate (LAL) assay**. The LAL test is a highly sensitive method used to detect **endotoxins (Lipopolysaccharide)** produced by **Gram-negative bacteria**. Among the options provided, **Neisseria meningitidis** is the only Gram-negative organism. **Analysis of Options:** * **Neisseria meningitidis (Correct):** A Gram-negative diplococcus. It is a leading cause of meningitis in young adults and is the only pathogen among the choices that would yield a positive LAL assay due to its endotoxin content [1]. * **Streptococcus pneumoniae (Incorrect):** While it is the most common cause of bacterial meningitis in adults, it is a **Gram-positive** coccus and lacks endotoxin; therefore, the LAL assay would be negative [1]. * **Staphylococcus aureus (Incorrect):** A **Gram-positive** coccus usually associated with post-neurosurgical procedures or endocarditis; LAL assay negative. * **Listeria monocytogenes (Incorrect):** A **Gram-positive** rod typically seen in neonates, elderly, or immunocompromised patients; LAL assay negative. **NEET-PG High-Yield Pearls:** * **LAL Assay:** Derived from the blood of the Horseshoe crab (*Limulus polyphemus*); used specifically to detect Gram-negative endotoxin. * **Drug of Choice:** For Meningococcal meningitis, IV Ceftriaxone is the standard treatment [1]. * **Prophylaxis:** Rifampicin is the drug of choice for close contacts of a patient with *N. meningitidis* [1]. * **CSF Findings in Bacterial Meningitis:** ↑ Proteins, ↓ Glucose (<40 mg/dL), ↑ Neutrophils (Polymorphs) [1].
Question 693: Pancytopenia is most common after which condition?
- A. Hepatitis (Correct Answer)
- B. Infective carditis
- C. Pyelonephritis
- D. Meningitis
Explanation: Explanation: The correct answer is **Hepatitis**. This refers to the clinical entity known as **Aplastic Anemia-Hepatitis Syndrome**. **1. Why Hepatitis is Correct:** Pancytopenia (a reduction in RBCs, WBCs, and platelets) is a well-recognized, severe complication of acute viral hepatitis [1]. It typically manifests as **Aplastic Anemia** occurring 2–3 months after an episode of acute hepatitis. While it can be associated with Hepatitis B and C, it is most frequently linked to **non-A, non-B, non-C, non-E (seronegative) hepatitis** [1]. The underlying mechanism is thought to be an immune-mediated destruction of hematopoietic stem cells by activated T-lymphocytes, triggered by the preceding viral infection. **2. Why Other Options are Incorrect:** * **Infective Endocarditis:** Typically presents with anemia of chronic disease and occasionally leukocytosis or thrombocytopenia (due to splenomegaly), but generalized bone marrow failure (pancytopenia) is not a standard feature. * **Pyelonephritis:** This is a localized bacterial infection of the kidney. It causes neutrophilic leukocytosis. It does not affect the bone marrow to cause pancytopenia. * **Meningitis:** Acute bacterial or viral meningitis causes pleocytosis in the CSF and systemic leukocytosis, but it does not lead to the suppression of all three cell lines. **High-Yield Clinical Pearls for NEET-PG:** * **Aplastic Anemia-Hepatitis Syndrome** is most common in young males. * The hepatitis is usually resolving or has resolved by the time pancytopenia develops. * **Other infections causing pancytopenia:** HIV, Parvovirus B19 (pure red cell aplasia or aplastic crisis in sickle cell), Tuberculosis (miliary), and Leishmaniasis (Kala-azar). * **Drug-induced pancytopenia:** Chloramphenicol is the classic board-exam culprit.
Question 694: Primary complex in which of the following sites suggests congenital tuberculosis?
- A. Lungs
- B. Liver (Correct Answer)
- C. Lymph nodes
- D. Skin
Explanation: The correct answer is **Liver**. **1. Why Liver is Correct:** Congenital tuberculosis occurs when the fetus is infected *in utero*. The most common route of transmission is **hematogenous**, via the **umbilical vein**. Blood from the placenta travels through the umbilical vein directly to the fetal liver. Therefore, the primary complex (Ghon complex) in congenital TB is characteristically located in the **liver** (specifically the porta hepatis), often accompanied by periportal lymphadenopathy. This is a pathognomonic finding that distinguishes congenital TB from postnatal infection. **2. Why Other Options are Incorrect:** * **Lungs:** This is the most common site for the primary complex in **postnatal (acquired) tuberculosis**, where the bacteria are inhaled. In these cases, organisms lodge in the alveoli and form a Ghon focus, which with regional hilar lymph nodes forms the primary complex [1]. While a fetus can develop lung lesions by aspirating infected amniotic fluid, the liver remains the classic site for hematogenous congenital transmission. * **Lymph Nodes:** While lymphadenopathy occurs as part of the primary complex (e.g., hilar nodes in lungs or periportal nodes in the liver), they are not the primary site of infection; they are a secondary component of the complex [2]. * **Skin:** Primary cutaneous TB is rare and usually occurs due to direct inoculation (e.g., accidental trauma or circumcision), not via congenital transmission. **3. High-Yield Clinical Pearls for NEET-PG:** * **Cantwell’s Criteria:** Used for the diagnosis of congenital TB. It requires the presence of TB lesions plus at least one of the following: 1) Primary liver complex, 2) Infection within the first week of life, 3) Exclusion of postnatal transmission. * **Route of Infection:** Most common is transplacental (umbilical vein); less common is aspiration of infected amniotic fluid. * **Clinical Presentation:** Often non-specific (fever, poor feeding, respiratory distress, hepatosplenomegaly) appearing 2–3 weeks after birth [3].
Question 695: What is the antibiotic of choice for the prophylaxis of endocarditis in adults undergoing dental procedures, considering penicillin allergy and inability to take oral medication?
- A. Clarithromycin 500 mg 1 hour before dental procedures
- B. Cephalexin 2 g 1 hour before dental procedures
- C. Cefadroxil 2 g 1 hour before dental procedures
- D. Clindamycin 600 mg 30 minutes before dental procedures (Correct Answer)
Explanation: **Explanation:** The primary objective of Infective Endocarditis (IE) prophylaxis is to prevent bacteremia (specifically *Viridans group streptococci*) during invasive dental procedures in high-risk patients. [1] **Why Option D is Correct:** According to the AHA/IDSA guidelines, for patients who are **allergic to Penicillin** and **unable to take oral medications**, the parenteral drug of choice is **Clindamycin (600 mg IV/IM)** administered 30–60 minutes before the procedure. Clindamycin provides excellent coverage against Gram-positive cocci and achieves high tissue concentrations. [2] (Note: While some recent guidelines have moved away from Clindamycin due to *C. difficile* concerns, it remains a classic standard in NEET-PG curriculum). **Analysis of Incorrect Options:** * **Option A (Clarithromycin):** This is an oral macrolide. While used for penicillin-allergic patients, it is incorrect here because the patient is specified as "unable to take oral medication." * **Options B & C (Cephalexin/Cefadroxil):** These are first-generation cephalosporins. They are contraindicated in patients with a history of **anaphylaxis, angioedema, or urticaria** related to penicillin due to the risk of cross-reactivity. [3] Furthermore, these are oral formulations. **High-Yield Clinical Pearls for NEET-PG:** * **Standard Prophylaxis:** Amoxicillin 2g PO (1 hour before). * **Unable to take Oral (No Allergy):** Ampicillin 2g IV/IM or Cefazolin/Ceftriaxone 1g IV/IM. * **Penicillin Allergy (Oral):** Cephalexin, Clindamycin, or Azithromycin/Clarithromycin. * **High-Risk Conditions requiring prophylaxis:** Prosthetic heart valves, prior IE, Cyanotic Congenital Heart Disease (unrepaired), and Cardiac transplant recipients with valve regurgitation. [1] * **Procedures:** Only those involving manipulation of gingival tissue, periapical region of teeth, or perforation of oral mucosa.
Question 696: A 33-year-old man has never been vaccinated for hepatitis B. Serologic tests reveal negative hepatitis B surface antigen (HBsAg) and positive antibody to surface antigen. Which of the following conditions does this serologic pattern best fit with?
- A. Previous hepatitis B infection (Correct Answer)
- B. Chronic active hepatitis
- C. Acute hepatitis B infection
- D. Poor prognosis
Explanation: ### Explanation The patient’s serologic profile shows **HBsAg negative** and **Anti-HBs positive**. This pattern indicates immunity to Hepatitis B. In the context of the options provided, this is most consistent with a **resolved previous infection** [1]. **1. Why Option A is Correct:** When a person recovers from a natural Hepatitis B infection, the surface antigen (HBsAg) disappears from the blood, and the body produces protective antibodies against it (Anti-HBs) [1]. Crucially, in a **previous infection**, the patient would also typically be positive for **Anti-HBc (IgG)** [1]. While the question doesn't mention Anti-HBc, the presence of Anti-HBs in a non-vaccinated individual is the hallmark of recovery and subsequent immunity. **2. Why the Other Options are Incorrect:** * **B. Chronic Active Hepatitis:** This would require **HBsAg** to be present for more than 6 months [1]. Anti-HBs is generally absent in chronic states as the virus has not been cleared. * **C. Acute Hepatitis B Infection:** The first marker to appear in acute infection is **HBsAg** [1]. Anti-HBs only appears during the convalescence phase after HBsAg has cleared. * **D. Poor Prognosis:** The presence of Anti-HBs signifies a **good prognosis**, as it indicates viral clearance and long-term immunity. **3. NEET-PG High-Yield Pearls:** * **Vaccination vs. Infection:** Both show positive Anti-HBs. However, **Vaccinated** patients are **Anti-HBc negative**, while those with **Previous Infection** are **Anti-HBc positive** [1]. * **Window Period:** This is the interval where HBsAg has disappeared but Anti-HBs has not yet appeared. The only marker present is **Anti-HBc IgM** [1]. * **HBeAg:** Indicates high viral replication and high infectivity. * **Anti-HBs:** The only serologic marker that confers immunity [1].
Question 697: A 47-year-old man suddenly develops high fever and hypotension. He has a generalized erythematous macular rash, and over the next day, develops gangrene of his left leg. Which of the following is the most likely organism?
- A. Corynebacterium diphtheriae
- B. Streptococcus group C
- C. Neisseria gonorrhoeae
- D. Streptococcus group A (Correct Answer)
Explanation: ### Explanation The clinical presentation of sudden high fever, hypotension (shock), a generalized erythematous macular rash (resembling sunburn), and rapidly progressing gangrene is classic for **Streptococcal Toxic Shock Syndrome (STSS)** and **Necrotizing Fasciitis** caused by **Streptococcus pyogenes (Group A Streptococcus - GAS)** [1]. **1. Why Streptococcus group A is correct:** GAS produces potent exotoxins (Superantigens like SpeA, SpeB, and SpeC) that cause massive, non-specific T-cell activation and cytokine release [1]. This leads to the "toxic shock" triad of fever, hypotension, and rash [2]. Furthermore, GAS is the most common cause of Type II Necrotizing Fasciitis, characterized by rapid tissue destruction and gangrene, often requiring surgical debridement. **2. Why the other options are incorrect:** * **Corynebacterium diphtheriae:** Typically presents with a greyish-white pseudomembrane in the pharynx and myocarditis/neuropathy due to its toxin. It does not cause rapid-onset gangrene or generalized macular rashes. * **Streptococcus group C:** While it can cause pharyngitis or skin infections, it rarely causes the fulminant toxic shock or necrotizing fasciitis seen with Group A [1]. * **Neisseria gonorrhoeae:** Disseminated gonococcal infection (DGI) presents with a triad of tenosynovitis, dermatitis (pustular or petechial lesions, not generalized erythema), and polyarthralgia. It does not cause gangrene. **Clinical Pearls for NEET-PG:** * **STSS vs. Staphylococcal TSS:** STSS (GAS) is frequently associated with a clear site of soft tissue infection (like necrotizing fasciitis) and bacteremia [1], whereas Staphylococcal TSS is often associated with tampon use or focal abscesses without bacteremia. * **Drug of Choice:** Penicillin G + Clindamycin. Clindamycin is added because it inhibits protein synthesis, thereby stopping the production of the offending exotoxins (the "Eagle Effect"). * **M-Protein:** The major virulence factor of GAS that inhibits phagocytosis.
Question 698: Prophylaxis for Pneumocystis jirovecii pneumonia is indicated in HIV-positive patients when the CD4 count is:
- A. <300 cells/mm3
- B. <200 cells/mm3 (Correct Answer)
- C. <100 cells/mm3
- D. <50 cells/mm3
Explanation: **Explanation:** *Pneumocystis jirovecii* pneumonia (PCP) is the most common opportunistic infection in patients with HIV/AIDS. The risk of developing PCP is inversely proportional to the CD4+ T-lymphocyte count [1]. **1. Why <200 cells/mm³ is correct:** The threshold of **<200 cells/mm³** (or a CD4 percentage <14%) is the standard clinical trigger for initiating primary prophylaxis [1]. At this level of immunosuppression, the host's cell-mediated immunity is sufficiently compromised to allow the reactivation or acquisition of *P. jirovecii*. The drug of choice for prophylaxis is **Trimethoprim-Sulfamethoxazole (TMP-SMX)** [1]. **2. Analysis of Incorrect Options:** * **<300 cells/mm³ (Option A):** At this level, the immune system is generally robust enough to prevent PCP; starting prophylaxis here would lead to unnecessary drug toxicity and resistance [1]. * **<100 cells/mm³ (Option C):** This is the threshold for initiating prophylaxis against **Toxoplasmosis** (if IgG positive) and **Cryptococcus** (pre-emptive screening). While PCP risk is very high here, prophylaxis should have already started at 200. * **<50 cells/mm³ (Option D):** This level indicates profound immunosuppression and is the threshold for **Mycobacterium avium complex (MAC)** prophylaxis (though routine MAC prophylaxis is now often deferred if ART is started immediately). **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice:** TMP-SMX (Double strength, once daily) [1]. * **Alternative if Sulfa-allergic:** Dapsone, Atovaquone, or Pentamidine (aerosolized) [1]. * **Discontinuation:** Prophylaxis can be safely stopped when the CD4 count rises to **>200 cells/mm³ for at least 3 months** in response to Antiretroviral Therapy (ART). * **Chest X-ray finding:** Classic "bat-wing" appearance or bilateral perihilar interstitial infiltrates [1]. * **Diagnosis:** Silver stain (Gomori Methenamine Silver) showing crushed ping-pong ball-shaped cysts [1].
Question 699: Fleeting skin lesions are often present in patients with:
- A. Viral hepatitis B (Correct Answer)
- B. Acute cholangitis
- C. Viral hepatitis A
- D. Carcinoma head of pancreas
Explanation: The correct answer is **Viral Hepatitis B**. **1. Why Viral Hepatitis B is correct:** Hepatitis B virus (HBV) infection often presents with a **prodromal (pre-icteric) phase** characterized by immune-complex-mediated phenomena [1]. This is known as the **Serum Sickness-like Syndrome**. It occurs due to the deposition of HBsAg-anti-HBs circulating immune complexes in small blood vessels and skin [1]. The classic triad includes fever, polyarthritis, and **fleeting skin lesions** (typically urticarial, maculopapular, or petechial rashes). These lesions are transient and usually disappear once clinical jaundice sets in [1]. **2. Why the other options are incorrect:** * **Viral Hepatitis A:** While it can cause systemic symptoms, it is rarely associated with immune-complex-mediated extrahepatic manifestations like fleeting rashes or arthritis compared to HBV [2]. * **Acute Cholangitis:** This typically presents with **Charcot’s Triad** (fever, jaundice, and RUQ pain). Skin manifestations are not a feature; if present, they would more likely be related to pruritus from obstructive jaundice, not fleeting rashes. * **Carcinoma Head of Pancreas:** This presents with painless, progressive obstructive jaundice and weight loss. Skin findings are limited to **pruritus** (due to bile salt deposition) and occasionally migratory thrombophlebitis (Trousseau sign), but not fleeting urticarial lesions. **3. NEET-PG High-Yield Pearls:** * **Serum Sickness-like Syndrome:** Seen in 10-20% of HBV patients during the prodrome. * **Extrahepatic Manifestations of HBV:** Polyarteritis Nodosa (PAN) and Membranous Glomerulonephritis. * **Extrahepatic Manifestations of HCV:** Mixed Cryoglobulinemia, Porphyria Cutanea Tarda, and Lichen Planus. * **Fleeting Rash:** Always think of HBV prodrome, Rheumatic Fever (Erythema Marginatum), or Still’s Disease in a clinical vignette.
Question 700: Actinomycosis is commonly seen in which of the following bones?
- A. Tibia
- B. Mandible (Correct Answer)
- C. Scapula
- D. Femur
Explanation: **Explanation:** **Actinomycosis** is a chronic, granulomatous infection caused by anaerobic, Gram-positive filamentous bacteria, most commonly *Actinomyces israelii*. These organisms are normal commensals of the oral cavity, gastrointestinal tract, and female genital tract. **Why the Mandible is Correct:** The most common clinical form of the disease is **Cervicofacial Actinomycosis** (accounting for ~50-60% of cases), often referred to as "Lumpy Jaw." The infection typically follows a breach in the oral mucosa due to dental caries, tooth extraction, or gingivitis. The bacteria spread by direct tissue contiguity rather than lymphatic or hematogenous routes. When the infection involves the bone, the **mandible** is the most frequently affected site, followed by the maxilla. It presents as a slow-growing, painless "woody" induration that eventually forms multiple external sinuses. **Why Other Options are Incorrect:** * **Tibia, Scapula, and Femur:** These are common sites for pyogenic osteomyelitis (usually caused by *Staphylococcus aureus*). While actinomycosis can rarely affect the extremities (Primary Actinomycosis of the Extremity) following trauma or human bites, it is statistically rare compared to the classic cervicofacial presentation. **High-Yield Clinical Pearls for NEET-PG:** * **Sulfur Granules:** The hallmark of actinomycosis is the presence of yellow, sand-like "sulfur granules" in the pus (actually masses of filamentous bacteria). * **Microscopy:** Shows Gram-positive branching filaments that are **non-acid fast** (distinguishing them from *Nocardia*, which is weakly acid-fast). * **Ray Fungus Appearance:** On histology, the colonies show a peripheral radiating arrangement of filaments. * **Treatment:** High-dose **Penicillin G** for a prolonged duration (6–12 months) is the treatment of choice.
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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