Infectious Diseases — MCQs
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Which of the following organisms would typically be found in a patient with atypical community-acquired pneumonia?
A 40-year-old woman, found to be positive for HBsAg, is excluded from blood donation due to a chronic shortage of blood. She is asymptomatic with normal liver function tests and negative serologic tests for IgM anti-HAV, anti-HBc, and anti-HCV. Repeat testing 6 months later shows the same results. Which of the following is the most appropriate statement regarding the pathophysiology of this patient's condition?
A 33-year-old woman presented with a 1-day history of fever and chills. On examination, she was febrile with a blood pressure of 70/40 mmHg. Over several hours, a widespread erythrodermic rash developed, and she collapsed. Further questioning revealed that the patient had removed a tampon shortly before presentation as she had just ceased menstruating. What is the most likely diagnosis?
A patient with AIDS is receiving therapy with Zidovudine, Lamivudine, and Indinavir. The patient develops pulmonary tuberculosis and requires treatment initiation. Which of the following medications should be avoided in this patient?
Which of the following is NOT a characteristic of secondary syphilis?
A young male admitted for pneumonia for 5 days develops gripping abdominal pain and loose stools. Which drug(s) might be beneficial in managing these new symptoms?
A 30-year-old patient presented with a 30-day history of jaundice. Liver function tests showed a bilirubin of 100mg/dL, SGOT/SGPT of 100/1450, and serum alkaline phosphatase of 240 IU. The patient was positive for HBsAg. What is the confirmatory test to establish acute Hepatitis B infection?
What is the diagnosis in a baby girl delivered at 30 weeks' gestation?
What is the diagnostic test of choice for neurosyphilis?
Most West Africans and people with origins in that region carry the Duffy-negative FyFy phenotype and are therefore resistant to______ malaria?
Infectious Diseases Indian Medical PG Practice Questions and MCQs
Question 101: Which of the following organisms would typically be found in a patient with atypical community-acquired pneumonia?
- A. Staphylococcus aureus
- B. Pseudomonas spp.
- C. Streptococcus pneumoniae
- D. Legionella pneumophila (Correct Answer)
Explanation: **Explanation:** Community-acquired pneumonia (CAP) is broadly classified into **Typical** and **Atypical** based on clinical presentation and causative organisms [1]. Atypical pneumonia is characterized by a subacute onset, non-productive cough, and systemic symptoms (headache, myalgia) that often seem "out of proportion" to the physical findings. **Why Legionella pneumophila is correct:** *Legionella pneumophila* is a classic "atypical" pathogen. These organisms are termed atypical because they cannot be visualized on Gram stain or cultured using standard agar. *Legionella* is often associated with contaminated water sources (AC cooling towers) [1] and presents with unique extrapulmonary features like diarrhea, hyponatremia, and elevated liver enzymes. **Analysis of Incorrect Options:** * **A. Staphylococcus aureus:** A typical pathogen often seen post-influenza or in IV drug users [1]. It typically causes necrotizing pneumonia with cavitary lesions. * **B. Pseudomonas spp.:** A common cause of Hospital-Acquired Pneumonia (HAP) or Ventilator-Associated Pneumonia (VAP) [2], particularly in patients with structural lung diseases like bronchiectasis or cystic fibrosis. * **C. Streptococcus pneumoniae:** The **most common cause** of typical CAP worldwide [1]. It presents classically with sudden onset high fever, productive cough (rusty sputum), and lobar consolidation [1]. **High-Yield NEET-PG Pearls:** 1. **Atypical Pathogens:** *Mycoplasma pneumoniae* (most common atypical), *Chlamydophila pneumoniae*, and *Legionella*. 2. **Diagnosis:** *Legionella* is best diagnosed via the **Urinary Antigen Test**. 3. **Treatment:** Atypical organisms lack a cell wall (or are intracellular); therefore, Beta-lactams are ineffective. **Macrolides** (Azithromycin) or **Fluoroquinolones** (Levofloxacin) are the drugs of choice. 4. **Radiology:** Atypical pneumonia often shows "patchy interstitial infiltrates" rather than lobar consolidation.
Question 102: A 40-year-old woman, found to be positive for HBsAg, is excluded from blood donation due to a chronic shortage of blood. She is asymptomatic with normal liver function tests and negative serologic tests for IgM anti-HAV, anti-HBc, and anti-HCV. Repeat testing 6 months later shows the same results. Which of the following is the most appropriate statement regarding the pathophysiology of this patient's condition?
- A. Chronic carrier state with no therapy indicated (Correct Answer)
- B. Clinically overt hepatitis will occur within 1 year
- C. Erroneous test results that need to be repeated
- D. Hepatitis B vaccination series is now required
Explanation: **Explanation:** This patient presents with a persistent **HBsAg positive** status for more than 6 months, which defines **Chronic Hepatitis B infection** [1]. However, her clinical profile—asymptomatic, normal liver function tests (LFTs), and lack of other viral markers—points toward an **Inactive HBsAg Carrier State**. **1. Why Option A is Correct:** A chronic carrier is defined by the presence of HBsAg in the serum for >6 months, normal serum aminotransferases (ALT/AST), and usually low or undetectable HBV DNA levels [1]. Since the patient has normal LFTs and no symptoms, she does not meet the criteria for active chronic hepatitis (which requires elevated ALT and high viral load). In the inactive carrier state, no antiviral therapy is indicated; management consists of periodic monitoring of LFTs and HBsAg status. **2. Why Incorrect Options are Wrong:** * **Option B:** Most inactive carriers remain stable for years. While reactivation can occur (especially under immunosuppression), it is not a rule that overt hepatitis will manifest within a year. * **Option C:** The results are consistent across two tests 6 months apart. This confirms chronicity rather than laboratory error [1]. * **Option D:** Hepatitis B vaccination is ineffective in individuals already infected with HBsAg [2]. Vaccination is a preventive measure for seronegative individuals. **NEET-PG High-Yield Pearls:** * **Chronic HBV Definition:** Persistence of HBsAg for >6 months [1]. * **Inactive Carrier Profile:** HBsAg (+), HBeAg (-), Anti-HBe (+), Normal ALT, and HBV DNA <2,000 IU/mL. * **Window Period:** The interval where HBsAg and Anti-HBs are both negative; **IgM anti-HBc** is the only diagnostic marker during this phase [1]. * **Blood Donation:** Any history of HBsAg positivity is a permanent deferral for blood donation, regardless of current LFTs.
Question 103: A 33-year-old woman presented with a 1-day history of fever and chills. On examination, she was febrile with a blood pressure of 70/40 mmHg. Over several hours, a widespread erythrodermic rash developed, and she collapsed. Further questioning revealed that the patient had removed a tampon shortly before presentation as she had just ceased menstruating. What is the most likely diagnosis?
- A. Haemolytic uraemic syndrome
- B. E. coli sepsis
- C. Fungal infection
- D. Toxic shock syndrome (Correct Answer)
Explanation: ### Explanation **Correct Answer: D. Toxic shock syndrome (TSS)** The clinical presentation is classic for **Staphylococcal Toxic Shock Syndrome**. The diagnosis is based on the rapid onset of high fever, hypotension (shock), and a diffuse erythrodermic rash (resembling a sunburn) that typically desquamates 1–2 weeks later. The mention of **tampon use** is a high-yield "buzzword" for TSS, as highly absorbent tampons provide a niche for *Staphylococcus aureus* to provide **TSST-1 (Toxic Shock Syndrome Toxin-1)**. This toxin acts as a **superantigen**, non-specifically cross-linking MHC II and T-cell receptors, leading to a massive "cytokine storm" (IL-1, IL-2, TNF-α, and IFN-γ) [1]. **Why the other options are incorrect:** * **A. Haemolytic uraemic syndrome (HUS):** Typically follows a prodrome of bloody diarrhea (EHEC O157:H7). It is characterized by the triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury, not an erythrodermic rash and sudden shock. * **B. E. coli sepsis:** While it can cause hypotension and fever, it does not typically present with a diffuse erythrodermic rash or a specific association with tampon use. * **C. Fungal infection:** Systemic fungal infections (like Candidemia) usually occur in immunocompromised patients and follow a more subacute course rather than this hyper-acute presentation. **NEET-PG High-Yield Pearls:** * **Causative Agent:** *S. aureus* (TSST-1) or *S. pyogenes* (Exotoxin A). Streptococcal TSS is more likely to have an associated necrotizing soft tissue infection [1]. * **Criteria:** Fever >38.9°C, SBP <90 mmHg, diffuse rash, and involvement of ≥3 organ systems (GI, Muscular, Renal, Hepatic, Hematologic, or CNS). * **Management:** Aggressive fluid resuscitation, removal of the source (tampon/packing), and antibiotics (Clindamycin is added to inhibit toxin production).
Question 104: A patient with AIDS is receiving therapy with Zidovudine, Lamivudine, and Indinavir. The patient develops pulmonary tuberculosis and requires treatment initiation. Which of the following medications should be avoided in this patient?
- A. Rifampicin (Correct Answer)
- B. Isoniazid
- C. Ethambutol
- D. Pyrazinamide
Explanation: **Explanation** The core concept in this question is the **drug-drug interaction** between Rifampicin and Protease Inhibitors (PIs) like Indinavir. **1. Why Rifampicin is the Correct Answer:** Rifampicin is a **potent inducer of the Cytochrome P450 (CYP3A4) enzyme** system in the liver. Indinavir, a Protease Inhibitor, is a substrate for the same enzyme. When co-administered, Rifampicin significantly accelerates the metabolism of Indinavir, leading to a **marked reduction (up to 80-90%) in its plasma concentration**. This results in sub-therapeutic levels of the ARV drug, leading to treatment failure and the development of drug-resistant HIV strains. Therefore, Rifampicin is contraindicated in patients taking PI-based regimens. **2. Why Other Options are Incorrect:** * **Isoniazid (B), Ethambutol (C), and Pyrazinamide (D):** These are standard first-line anti-tubercular drugs (ATT) that do not have significant metabolic interactions with Zidovudine, Lamivudine, or Indinavir [1]. They can be safely used in HIV-TB co-infected patients. **3. High-Yield Clinical Pearls for NEET-PG:** * **Alternative:** If a Rifamycin must be used with a Protease Inhibitor, **Rifabutin** is the preferred choice because it is a much weaker inducer of CYP3A4 compared to Rifampicin. * **Efavirenz (NNRTI):** Unlike PIs, Efavirenz can be used with Rifampicin, though a dose adjustment (increasing Efavirenz to 800mg) was previously suggested (current guidelines often maintain 600mg). * **Integrase Inhibitors:** If a patient is on **Dolutegravir**, the dose must be increased to **50mg twice daily** (instead of once daily) if Rifampicin is co-administered. * **Rule of Thumb:** Rifampicin "Ramps up" enzymes (inducer); Cimetidine/Ketoconazole "Kicks them down" (inhibitors).
Question 105: Which of the following is NOT a characteristic of secondary syphilis?
- A. Occurs 6-8 weeks after the development of the chancre
- B. Painful and itchy maculopapular lesions (Correct Answer)
- C. Constitutional features are present
- D. Meningitis, hepatitis, glomerulonephritis, and uveitis may occur
Explanation: Syphilis, caused by the spirochete *Treponema pallidum*, is a multi-stage systemic infection. Understanding the clinical presentation of each stage is crucial for NEET-PG. **Why Option B is the Correct Answer:** The hallmark of secondary syphilis is a generalized **non-pruritic (not itchy)** and **painless** maculopapular rash [1]. The rash characteristically involves the **palms and soles**, a high-yield clinical sign [1]. Because the question asks for the "NOT" characteristic, Option B is correct because syphilitic lesions are typically asymptomatic [1]. **Analysis of Incorrect Options:** * **Option A:** Secondary syphilis typically manifests **6 to 8 weeks** (range 2–12 weeks) after the appearance of the primary chancre. In some cases, the chancre may still be healing when the secondary stage begins. * **Option C:** Unlike the primary stage, secondary syphilis is a systemic illness. Patients frequently present with **constitutional symptoms** such as malaise, fever, sore throat, weight loss, and generalized lymphadenopathy (especially epitrochlear nodes). * **Option D:** This stage represents hematogenous dissemination. While rare, it can lead to **organ-specific complications** including aseptic meningitis, hepatitis (with disproportionately high alkaline phosphatase), immune-complex glomerulonephritis (nephrotic syndrome), and ocular involvement like uveitis. **Clinical Pearls for NEET-PG:** * **Condyloma Lata:** Highly infectious, moist, wart-like papules found in intertriginous areas (e.g., axilla, perineum) during the secondary stage [1]. * **Lues Maligna:** A severe, pleomorphic form of secondary syphilis with necrotic lesions, usually seen in HIV-positive patients. * **Diagnosis:** Non-specific treponemal tests (VDRL/RPR) have their highest titers during the secondary stage. If the titer is extremely high, a "Prozone phenomenon" may cause a false negative.
Question 106: A young male admitted for pneumonia for 5 days develops gripping abdominal pain and loose stools. Which drug(s) might be beneficial in managing these new symptoms?
- A. Capsule Imodium 4-8 mg/day
- B. Tablet Ciprofloxacin 500 mg twice daily (Correct Answer)
- C. Oral Metronidazole
- D. Capsule Diphenoxylate
Explanation: The clinical scenario describes a patient hospitalized for pneumonia who develops new-onset abdominal pain and diarrhea after 5 days of antibiotic therapy [1]. This is a classic presentation of **Antibiotic-Associated Diarrhea (AAD)**, most commonly caused by *Clostridioides difficile* (formerly *Clostridium difficile*). **Why Option B is Correct:** While **Oral Vancomycin** or **Fidaxomicin** are currently the first-line treatments for *C. difficile* infection (CDI) according to recent IDSA guidelines, **Metronidazole** was historically the drug of choice for mild-to-moderate cases [1]. However, in the context of this specific question (often based on older clinical patterns or specific examiner preferences in Indian PG exams), **Ciprofloxacin** [2] is sometimes discussed in the context of treating other bacterial superinfections or specific enteric pathogens. *Note: In modern clinical practice, if CDI is suspected, Metronidazole or Vancomycin is preferred. However, based on the provided key, Ciprofloxacin is the designated answer, likely targeting a broader bacterial enteritis or specific institutional protocol.* **Why Other Options are Incorrect:** * **Options A & D (Imodium/Loperamide and Diphenoxylate):** These are anti-motility agents. They are **contraindicated** in infectious diarrhea (especially CDI) because they inhibit the clearance of toxins from the bowel, potentially leading to **Toxic Megacolon** and perforation [1]. * **Option C (Oral Metronidazole):** While clinically appropriate for CDI, it is not the marked answer here. In many NEET-PG questions, if a patient develops diarrhea post-antibiotics, the examiner is testing your knowledge of avoiding anti-motility agents and selecting an appropriate antimicrobial. **High-Yield Clinical Pearls for NEET-PG:** * **Most common cause of AAD:** *C. difficile*. * **Most common antibiotic causing CDI:** Historically Clindamycin; currently, Fluoroquinolones, Cephalosporins, and Penicillins are frequent triggers [2]. * **Diagnosis:** Stool assay for Toxin A and B or GDH antigen [1]. * **Endoscopy finding:** Yellowish-white plaques on colonic mucosa (**Pseudomembranous colitis**). * **Treatment of choice (Current):** Oral Vancomycin (125 mg QID) [2].
Question 107: A 30-year-old patient presented with a 30-day history of jaundice. Liver function tests showed a bilirubin of 100mg/dL, SGOT/SGPT of 100/1450, and serum alkaline phosphatase of 240 IU. The patient was positive for HBsAg. What is the confirmatory test to establish acute Hepatitis B infection?
- A. IgM Anti-HBc antibody (Correct Answer)
- B. HBeAg
- C. HBV DNA by PCR
- D. Anti-HBc antibody
Explanation: ### Explanation The clinical presentation of prolonged jaundice, significantly elevated transaminases (SGPT > SGOT), and positive HBsAg strongly suggests acute viral hepatitis. To differentiate between an acute infection and a flare of chronic Hepatitis B, specific serological markers are required. **1. Why IgM Anti-HBc is the Correct Answer:** The **IgM antibody to Hepatitis B core antigen (IgM anti-HBc)** is the gold standard for diagnosing **acute Hepatitis B infection** [1]. It appears shortly after HBsAg and persists for about 6–12 months [2]. Crucially, it is the only marker present during the **"Window Period"** (the gap between the disappearance of HBsAg and the appearance of Anti-HBs). Its presence confirms that the infection is new rather than a chronic carrier state. **2. Why the Other Options are Incorrect:** * **HBeAg:** This is a marker of **active viral replication** and high infectivity [1]. While often present in acute phases, it does not distinguish between acute infection and a highly replicative chronic state. * **HBV DNA by PCR:** This measures the **viral load** [1]. While useful for monitoring treatment and assessing infectivity, it is not the primary tool for diagnosing "acuteness," as high levels can be seen in both acute and chronic phases. * **Anti-HBc antibody (Total):** This test measures both IgM and IgG. Since **IgG anti-HBc** persists for life after any exposure, a total antibody test cannot differentiate between a past, chronic, or current acute infection [2]. **Clinical Pearls for NEET-PG:** * **HBsAg:** First marker to appear in blood (as early as 1–2 weeks after exposure). The persistence of HBsAg for longer than 6 months indicates chronic infection [1]. * **Window Period:** HBsAg (-), Anti-HBs (-), **IgM Anti-HBc (+)**. * **Chronic Infection:** Defined by the persistence of HBsAg for >6 months [1]. * **Immunity:** Presence of **Anti-HBs** indicates immunity (either via recovery or vaccination). If Anti-HBs is positive but Anti-HBc is negative, the patient was vaccinated [2].
Question 108: What is the diagnosis in a baby girl delivered at 30 weeks' gestation?
- A. Congenital cytomegalovirus infection (Correct Answer)
- B. Ecthyma gangrenosum
- C. Homozygous protein C deficiency
- D. Klippel-Trenaunay syndrome
Explanation: ***Congenital cytomegalovirus infection*** - The classic **blueberry muffin rash** (purpuric papules) in a preterm neonate is the hallmark of congenital CMV infection. - CMV is the most common **intrauterine infection** and often presents with **IUGR**, **hepatosplenomegaly**, and **thrombocytopenia** in premature infants. *Ecthyma gangrenosum* - Characterized by **necrotic ulcerative lesions** with black eschar centers, typically caused by **Pseudomonas aeruginosa**. - Occurs in **immunocompromised patients** with **septicemia**, not as a congenital condition in neonates. *Homozygous protein C deficiency* - Presents with **purpura fulminans** causing extensive skin necrosis and **warfarin-induced skin necrosis**. - Associated with **thrombotic complications** rather than the blueberry muffin appearance seen in CMV. *Klippel-Trenaunay syndrome* - Features a **port-wine stain**, **varicose veins**, and **limb hypertrophy** (capillary-lymphatic-venous malformation). - This is a **vascular malformation syndrome**, not an infectious process presenting with systemic illness.
Question 109: What is the diagnostic test of choice for neurosyphilis?
- A. VDRL (Correct Answer)
- B. TA-ABS
- C. TPI
- D. RPR
Explanation: The diagnosis of neurosyphilis relies on the examination of Cerebrospinal Fluid (CSF). The **CSF-VDRL** (Venereal Disease Research Laboratory) test is the **diagnostic test of choice** because it is highly specific [2]. While it has low sensitivity (meaning a negative result does not rule out neurosyphilis), a **positive CSF-VDRL** in the absence of gross blood contamination is considered pathognomonic and diagnostic of neurosyphilis. **Analysis of Options:** * **A. VDRL (Correct):** It is the gold standard for CSF testing due to its high specificity [2]. * **B. FTA-ABS (Fluorescent Treponemal Antibody Absorption):** This is a treponemal test. While highly sensitive in CSF (a negative result helps rule out neurosyphilis), it is not the "test of choice" for diagnosis because it remains positive for life and lacks the specificity of VDRL [2]. * **C. TPI (Treponema Pallidum Immobilization):** Historically the gold standard for syphilis diagnosis, it is now obsolete, expensive, and rarely used in modern clinical practice. * **D. RPR (Rapid Plasma Reagin):** While used for screening systemic syphilis, RPR is **not performed on CSF** because it is not standardized for this purpose and lacks the necessary sensitivity/specificity for neuro-diagnosis [2]. **High-Yield Clinical Pearls for NEET-PG:** * **Screening Test for Syphilis:** RPR or VDRL (Nontreponemal) [2]. * **Confirmatory Test for Syphilis:** FTA-ABS or TP-PA (Treponemal) [2]. * **Neurosyphilis Screening:** If a patient has neurological symptoms and a reactive serum treponemal test, a lumbar puncture is mandatory [1]. * **Treatment of Choice:** Aqueous Penicillin G (18–24 million units per day, IV) for 10–14 days. Procaine Penicillin is an alternative but requires concurrent Probenecid.
Question 110: Most West Africans and people with origins in that region carry the Duffy-negative FyFy phenotype and are therefore resistant to______ malaria?
- A. P. vivax (Correct Answer)
- B. P. ovale
- C. P. malariae
- D. P. falciparum
Explanation: Most West Africans and people with origins in that region carry the Duffy-negative FyFy phenotype and are therefore resistant to P. vivax malaria. **Explanation:** The correct answer is **A. P. vivax**. **Mechanism of Resistance:** The invasion of human erythrocytes by *Plasmodium vivax* requires a specific interaction between the parasite’s Duffy-binding protein and the **Duffy Antigen Receptor for Chemokines (DARC)** on the surface of the red blood cell (RBC). Individuals with the **FyFy phenotype** (Duffy-negative) lack these receptors on their RBCs. Consequently, the *P. vivax* merozoite cannot form a junction with the cell membrane or enter the erythrocyte. This phenotype is highly prevalent in West and Central Africa (up to 95-99%), providing natural selection-driven resistance against *P. vivax* malaria. **Analysis of Incorrect Options:** * **B. P. ovale:** While *P. ovale* is common in Africa, it uses different, non-Duffy-dependent receptors for RBC entry. Therefore, Duffy-negative individuals remain susceptible to *P. ovale* [1]. * **C. P. malariae:** This species utilizes different pathways for invasion and is not dependent on the Duffy antigen [1]. * **D. P. falciparum:** This is the most lethal species in Africa. It uses multiple alternative pathways (e.g., Glycophorins A, B, and C) to enter RBCs. Resistance to *P. falciparum* is typically associated with **Sickle Cell Trait (HbAS)**, G6PD deficiency, or Thalassemia, rather than the Duffy status. **High-Yield Clinical Pearls for NEET-PG:** * **Duffy Antigen:** Also known as the **Fy glycoprotein**. * **Duffy-Negative Status:** Protects against *P. vivax* but **not** against *P. knowlesi* (which also uses Duffy receptors in macaques but can use others in humans). * **Vivax in Africa:** Because of the FyFy phenotype, *P. vivax* is virtually absent in indigenous West African populations [1]. * **Schüffner’s Dots:** Characteristically seen in RBCs infected with *P. vivax* and *P. ovale* [1].
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