Infectious Diseases Practice Questions

Q: A 22-year-old man consumed a beef sandwich and potato salad, experiencing vomiting 2 hours after the meal. What is the probable diagnosis?

Staphylococcus aureus poisoning. **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.

Q: 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?

Neisseria meningitidis. **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].

Q: Pancytopenia is most common after which condition?

Hepatitis. 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.

Q: 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?

Clindamycin 600 mg 30 minutes before dental procedures. **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.

Q: 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?

Previous hepatitis B infection. ### 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].

Q: Prophylaxis for Pneumocystis jirovecii pneumonia is indicated in HIV-positive patients when the CD4 count is:

<200 cells/mm3. **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].

Q: Fleeting skin lesions are often present in patients with:

Viral hepatitis B. 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.

Q: Actinomycosis is commonly seen in which of the following bones?

Mandible. **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.

Question 1

A 22-year-old man consumed a beef sandwich and potato salad, experiencing vomiting 2 hours after the meal. What is the probable diagnosis?

Accepted Answer

Staphylococcus aureus poisoning

Answer

Bacillus cereus poisoning

Answer

Clostridial poisoning

Answer

Helicobacter pylori poisoning

Question 2

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?

Accepted Answer

Neisseria meningitidis

Answer

Staphylococcus aureus

Answer

Streptococcus pneumoniae

Answer

Listeria monocytogenes

Question 3

Pancytopenia is most common after which condition?

Accepted Answer

Hepatitis

Answer

Infective carditis

Answer

Pyelonephritis

Answer

Meningitis

Question 4

Primary complex in which of the following sites suggests congenital tuberculosis?

Accepted Answer

Liver

Answer

Lungs

Answer

Lymph nodes

Answer

Skin

Question 5

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?

Accepted Answer

Clindamycin 600 mg 30 minutes before dental procedures

Answer

Clarithromycin 500 mg 1 hour before dental procedures

Answer

Cephalexin 2 g 1 hour before dental procedures

Answer

Cefadroxil 2 g 1 hour before dental procedures

Question 6

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?

Accepted Answer

Previous hepatitis B infection

Answer

Chronic active hepatitis

Answer

Acute hepatitis B infection

Answer

Poor prognosis

Question 7

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?

Accepted Answer

Streptococcus group A

Answer

Corynebacterium diphtheriae

Answer

Streptococcus group C

Answer

Neisseria gonorrhoeae

Question 8

Prophylaxis for Pneumocystis jirovecii pneumonia is indicated in HIV-positive patients when the CD4 count is:

Accepted Answer

<200 cells/mm3

Answer

<300 cells/mm3

Answer

<100 cells/mm3

Answer

<50 cells/mm3

Question 9

Fleeting skin lesions are often present in patients with:

Accepted Answer

Viral hepatitis B

Answer

Acute cholangitis

Answer

Viral hepatitis A

Answer

Carcinoma head of pancreas

Question 10

Actinomycosis is commonly seen in which of the following bones?

Accepted Answer

Mandible

Answer

Tibia

Answer

Scapula

Answer

Femur

Infectious Diseases — MCQs

Infectious Diseases — MCQs

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