Infectious Diseases — MCQs

A 30-year-old pregnant woman develops severe sepsis with headache and neck stiffness after eating coleslaw, soft cheese, and cantaloupes at a potluck dinner. Two other women who attended the event develop self-limiting abdominal cramps, nausea, and vomiting. Which infectious agent is most likely responsible?

Q116Easy

What is the empirical antibiotic treatment for a suspected case of pneumonia in an outpatient who has not received antibiotics in the past three months?

Q117Medium

A dipstick kit was found to be positive. What is the treatment of choice?

Q118Medium

A farmer from Central Asia presents with an acute onset of swinging pyrexia, rigor, sweating, and monoarticular arthritis of the hip. He also complained of headache and insomnia. On examination, he is found to have small firm splenomegaly and hepatomegaly. His counts showed leucopenia with relative lymphocytosis. What is the most likely diagnosis?

Q119Medium

Which of the following is NOT true about fluorescent antibody detection tests in the diagnosis of Plasmodium falciparum?

Q120Easy

What is the most common diagnostic test for HIV infection?

Infectious Diseases Indian Medical PG Practice Questions and MCQs

Question 111: What is the most common cause of surgical wound infection?

A. Staphylococcus aureus (Correct Answer)
B. Streptococcus
C. Pseudomonas
D. Anaerobes

Explanation: **Explanation:** **Surgical Site Infections (SSIs)** are a significant cause of postoperative morbidity. The correct answer is **Staphylococcus aureus** because it is the most common pathogen isolated from surgical wounds across almost all categories of surgery. 1. **Why Staphylococcus aureus is correct:** * *S. aureus* is a ubiquitous commensal found on human skin and in the anterior nares [1]. * During an incision, these bacteria are introduced from the patient’s own skin flora (endogenous source) or from the hands of healthcare workers (exogenous source) into the deeper tissues. * Its ability to produce biofilms and various virulence factors (like toxins and enzymes) allows it to colonize surgical hardware and damaged tissue effectively [1]. 2. **Why other options are incorrect:** * **Streptococcus:** While *Streptococcus pyogenes* can cause aggressive necrotizing infections, it is less frequently isolated than *S. aureus* in routine postoperative wounds. * **Pseudomonas:** This is a common cause of **nosocomial (hospital-acquired) infections**, particularly in burn victims or patients with prolonged ICU stays, but it is not the overall leading cause of SSIs. * **Anaerobes:** These are common causes of infection only following surgeries involving "dirty" or contaminated sites, such as the colon or female genital tract (e.g., *Bacteroides fragilis*). **High-Yield Clinical Pearls for NEET-PG:** * **Most common organism overall:** *Staphylococcus aureus* [1]. * **Most common organism in prosthetic valve endocarditis (early):** *Staphylococcus epidermidis*. * **Timing:** SSIs typically occur within 30 days of surgery (or up to 1 year if a prosthetic implant is involved). * **Prevention:** The most effective measure to prevent SSI is the administration of a prophylactic antibiotic (usually a 1st or 2nd generation Cephalosporin) within **60 minutes before** the skin incision.

Question 112: Which statement is not true regarding Yaws?

A. It is spread by sexual transmission. (Correct Answer)
B. It is caused by Treponema pallidum pertenue.
C. It exhibits cross-immunity with syphilis.
D. It cannot be differentiated serologically from Treponema pallidum.

Explanation: **Explanation:** **1. Why Option A is the Correct Answer (The "Not True" Statement):** Yaws is a non-venereal (endemic) treponematosis. Unlike syphilis, it is **not spread by sexual transmission**. It is primarily transmitted through **direct skin-to-skin contact** with the fluid from early skin lesions, often facilitated by minor abrasions or trauma. It is predominantly a disease of children living in poor, humid, tropical regions. **2. Analysis of Other Options:** * **Option B:** Yaws is indeed caused by *Treponema pallidum* subspecies *pertenue*. This organism is morphologically identical to the subspecies that causes syphilis (*T. pallidum pallidum*). * **Option C:** There is a significant degree of **cross-immunity** between yaws and syphilis. Infection with yaws can provide partial protection against subsequent infection with venereal syphilis, which is why the two diseases rarely coexist in the same population. * **Option D:** Because the subspecies are genetically and morphologically similar, they elicit the same antibody response. Therefore, yaws **cannot be differentiated** from syphilis using standard serological tests (both non-treponemal like VDRL/RPR and treponemal like TPHA/FTA-ABS). **Clinical Pearls for NEET-PG:** * **Primary Lesion:** Known as the "Mother Yaws" (a painless, itchy papule/papilloma). * **Secondary Stage:** Characterized by "Crab Yaws" (painful papillomas on soles leading to a crab-like gait). * **Tertiary Stage:** Can cause "Goundou" (paranasal swelling) and "Gangosa" (destructive rhinopharyngitis). * **Treatment:** A single dose of **Oral Azithromycin** is now the preferred treatment (WHO Morges Strategy), replacing injectable Penicillin G Benzathine. * **India Status:** India was declared free of Yaws by the WHO in 2016.

Question 113: Duffy blood group antigen negativity confers protection against infection by which Plasmodium species?

A. Plasmodium falciparum
B. Plasmodium ovale
C. Plasmodium vivax (Correct Answer)
D. Plasmodium malariae

Explanation: ### Explanation **Correct Answer: C. Plasmodium vivax** **Mechanism of Protection:** *Plasmodium vivax* requires the **Duffy Antigen Receptor for Chemokines (DARC)**, located on the surface of red blood cells (RBCs), to act as a receptor for its **Duffy Binding Protein (DBP)**. This interaction is essential for the merozoite to invade the erythrocyte. Individuals who are **Duffy-negative** (genotype *Fy/Fy*) lack these receptors on their RBCs, making the cells resistant to invasion by *P. vivax*. This phenotype is highly prevalent in West African populations, explaining why *P. vivax* malaria is rare in that region. **Analysis of Incorrect Options:** * **A. Plasmodium falciparum:** Uses multiple alternative receptors (e.g., Glycophorins A, B, and C) for invasion. Protection against *P. falciparum* is instead associated with Sickle Cell Trait (HbAS), G6PD deficiency, and Thalassemia [1]. * **B. Plasmodium ovale:** Primarily uses different, less-defined pathways for entry and is common in West Africa, often "replacing" the niche left by *P. vivax* in Duffy-negative populations. * **D. Plasmodium malariae:** Does not utilize the Duffy antigen for RBC entry; its specific receptor mechanisms are distinct from those of *P. vivax* [1]. **High-Yield Clinical Pearls for NEET-PG:** * **Duffy Status:** Duffy-negative individuals can still be infected by *P. vivax* in rare cases (recent reports from Ethiopia/Madagascar), but for exam purposes, it remains the classic protective factor. * **Reticulocyte Preference:** *P. vivax* and *P. ovale* selectively infect young RBCs (reticulocytes), whereas *P. falciparum* infects RBCs of all ages (leading to high parasitemia) [1]. * **Schüffner’s Dots:** Characteristically seen in RBCs infected with *P. vivax* and *P. ovale*. * **Hypnozoites:** Only *P. vivax* and *P. ovale* form dormant liver stages (hypnozoites), requiring **Primaquine** or **Tafenoquine** for a radical cure [1].

Question 114: What is the most common cause of community-acquired pneumonia?

A. Streptococcus pneumoniae (Correct Answer)
B. Klebsiella pneumoniae
C. Vibrio cholerae
D. Haemophilus influenzae

Explanation: **Explanation:** **Streptococcus pneumoniae** (Pneumococcus) remains the **most common cause** of Community-Acquired Pneumonia (CAP) worldwide across all age groups [1]. It is a Gram-positive, lancet-shaped diplococcus. The underlying medical concept is its high prevalence in the nasopharyngeal flora and its potent polysaccharide capsule, which allows it to evade host phagocytosis, leading to classic lobar pneumonia. **Analysis of Incorrect Options:** * **Klebsiella pneumoniae:** While a significant cause of CAP, it is typically associated with specific risk factors such as **chronic alcoholism**, diabetes mellitus, or nursing home residents [2]. It characteristically produces "currant jelly" sputum and affects the upper lobes. * **Vibrio cholerae:** This is a gastrointestinal pathogen responsible for Cholera (profuse watery diarrhea). It is not a respiratory pathogen and does not cause pneumonia. * **Haemophilus influenzae:** This is the **second most common** cause of CAP. It is particularly prevalent in patients with underlying **COPD** or structural lung diseases (like bronchiectasis). **High-Yield Clinical Pearls for NEET-PG:** * **Most common atypical pneumonia:** *Mycoplasma pneumoniae* (often presents with "walking pneumonia" and bullous myringitis). * **Post-viral (Influenza) pneumonia:** Most commonly caused by *Staphylococcus aureus* [1]. * **Rust-colored sputum:** Pathognomonic for *Streptococcus pneumoniae* [2]. * **Diagnostic Gold Standard:** Presence of an infiltrate on Chest X-ray. * **CURB-65 Score:** Used to decide the site of care (outpatient vs. inpatient) for CAP patients [2].

Question 115: A 30-year-old pregnant woman develops severe sepsis with headache and neck stiffness after eating coleslaw, soft cheese, and cantaloupes at a potluck dinner. Two other women who attended the event develop self-limiting abdominal cramps, nausea, and vomiting. Which infectious agent is most likely responsible?

A. Herpes simplex virus
B. Epstein-Barr virus
C. Parvovirus B19
D. Listeria monocytogenes (Correct Answer)

Explanation: The clinical presentation is classic for **Listeriosis**, caused by *Listeria monocytogenes*. This gram-positive, facultative intracellular bacillus is a common cause of foodborne illness associated with contaminated **deli meats, soft cheeses (brie, camembert), coleslaw, and unwashed produce (cantaloupes).** 1. **Why Listeria is correct:** * **Vulnerable Populations:** It primarily affects pregnant women (due to altered cell-mediated immunity), neonates, and the elderly. * **Clinical Spectrum:** In healthy individuals, it causes self-limiting febrile gastroenteritis (as seen in the two other women) [1]. However, in pregnant women, it can lead to severe sepsis and has a high tropism for the CNS, causing **meningitis** (headache and neck stiffness). It is a leading cause of neonatal sepsis and meningitis via transplacental transmission. 2. **Why other options are incorrect:** * **HSV & EBV:** These viruses do not typically present as foodborne outbreaks. While HSV can cause encephalitis, it is not associated with the consumption of specific foods or cluster outbreaks. * **Parvovirus B19:** This causes Erythema Infectiosum (Fifth disease) and can lead to hydrops fetalis in pregnancy, but it does not cause acute sepsis or meningitis following food ingestion. **High-Yield Clinical Pearls for NEET-PG:** * **Microscopy:** Exhibits characteristic **"Tumbling Motility"** at 25°C. * **Cold Enrichment:** It can grow at temperatures as low as 4°C (refrigeration doesn't prevent growth). * **Treatment of Choice:** **Ampicillin** (Listeria is inherently resistant to all cephalosporins). * **Pathogenesis:** Uses **Actin tails** (actin rockets) for cell-to-cell spread, avoiding the host's humoral immune response.

Question 116: What is the empirical antibiotic treatment for a suspected case of pneumonia in an outpatient who has not received antibiotics in the past three months?

A. Oral Clarithromycin (Correct Answer)
B. Oral fluoroquinolone
C. Oral beta-lactam with injectable aminoglycoside
D. Oral linezolid

Explanation: The management of Community-Acquired Pneumonia (CAP) is guided by the patient's previous health status and risk factors for drug resistance. According to the **ATS/IDSA guidelines**, the primary goal in outpatient CAP is to cover common pathogens like *Streptococcus pneumoniae* and atypical organisms (*Mycoplasma pneumoniae*, *Chlamydophila pneumoniae*). **Why Option A is Correct:** In a previously healthy outpatient with no recent antibiotic use (past 3 months) and no risk factors for MRSA or *Pseudomonas*, **Macrolides** (e.g., Clarithromycin or Azithromycin) [1] or **Doxycycline** are the first-line empirical treatments. They provide excellent coverage against both typical and atypical pathogens in this specific patient profile. **Why Other Options are Incorrect:** * **B. Oral Fluoroquinolone:** Respiratory quinolones (e.g., Levofloxacin, Moxifloxacin) [1] are reserved for patients with comorbidities (heart/lung/liver disease), recent antibiotic use, or those who have failed first-line therapy. Overuse is avoided to prevent resistance and TB masking. * **C. Oral Beta-lactam with Injectable Aminoglycoside:** This is inappropriate for outpatient care. Beta-lactams alone lack atypical coverage, and aminoglycosides have poor lung penetration and high toxicity, making them unsuitable for routine CAP. * **D. Oral Linezolid:** This is a "reserve" antibiotic used specifically for MRSA or Vancomycin-resistant Enterococci (VRE) [2]. It is never used as empirical therapy for uncomplicated CAP. **High-Yield Clinical Pearls for NEET-PG:** * **CURB-65 Score:** Used to decide the site of care (Outpatient vs. Inpatient) [3]. A score of 0-1 usually indicates outpatient management. * **Drug of Choice:** For outpatients *with* comorbidities, the treatment is a combination of a Beta-lactam (Amoxicillin/Clavulanate) **plus** a Macrolide, OR a Respiratory Fluoroquinolone alone. * **Atypical Pneumonia:** Suspect if the patient has a non-productive cough and "walking pneumonia" presentation; Macrolides are the treatment of choice [1].

Question 117: A dipstick kit was found to be positive. What is the treatment of choice?

A. Amphotericin B (Correct Answer)
B. Albendazole
C. Doxycycline
D. Praziquantel

Explanation: ***Amphotericin B*** - **Liposomal Amphotericin B** is the first-line treatment for **visceral leishmaniasis** (Kala-azar) as per **NVBDCP guidelines** in India. - The **rK39 dipstick test** is highly specific for visceral leishmaniasis, and amphotericin B has excellent efficacy with fewer side effects than conventional treatments. *Albendazole* - This is an **antihelminthic drug** used to treat parasitic worm infections like **roundworms**, **hookworms**, and **tapeworms**. - It has **no activity against Leishmania parasites**, which are protozoa causing visceral leishmaniasis. *Doxycycline* - This is a **tetracycline antibiotic** effective against bacterial infections and some **rickettsial diseases** like scrub typhus. - It has **no anti-leishmanial activity** and cannot treat the protozoan infection causing positive rK39 test. *Praziquantel* - This is an **antihelminthic drug** specifically used to treat **schistosomiasis** and other **trematode infections**. - It is completely **ineffective against Leishmania** parasites and would not treat visceral leishmaniasis.

Question 118: A farmer from Central Asia presents with an acute onset of swinging pyrexia, rigor, sweating, and monoarticular arthritis of the hip. He also complained of headache and insomnia. On examination, he is found to have small firm splenomegaly and hepatomegaly. His counts showed leucopenia with relative lymphocytosis. What is the most likely diagnosis?

A. Brucellosis (Correct Answer)
B. Bagassosis
C. Byssionosis
D. Chikungunya fever

Explanation: ### **Explanation** The clinical presentation is classic for **Brucellosis** (Malta Fever/Undulant Fever), a zoonotic infection common among farmers, veterinarians, and those consuming unpasteurized dairy. **Why Brucellosis is correct:** * **Clinical Triad:** The combination of **swinging pyrexia** (undulant fever), **profuse sweating** (often with a characteristic moldy odor), and **joint involvement** is hallmark. [1] * **Musculoskeletal Features:** Sacroiliitis and **monoarticular arthritis** (commonly the hip or knee) are the most frequent localized complications. [2] * **Systemic Signs:** The pathogen (Brucella species) targets the reticuloendothelial system, leading to **hepatosplenomegaly** and lymphadenopathy. * **Hematology:** Characteristically shows **leucopenia with relative lymphocytosis**, reflecting the intracellular nature of the bacteria. * **Epidemiology:** The patient’s occupation (farmer) and geography (Central Asia) are high-risk factors for Brucella exposure. **Why other options are incorrect:** * **Bagassosis & Byssionosis:** These are occupational lung diseases (pneumoconioses) caused by inhaling organic dust (sugarcane fiber and cotton, respectively). They present with respiratory symptoms (dyspnea, cough) rather than fever, arthritis, and splenomegaly. * **Chikungunya fever:** While it causes high fever and arthritis, it typically presents with **polyarthritis** (small joints of hands/feet) and a prominent maculopapular rash, without significant hepatosplenomegaly or leucopenia. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Bone marrow culture (more sensitive than blood culture). [2] * **Standard Serology:** Standard Agglutination Test (SAT); titers >1:160 are significant. [2] * **Treatment:** WHO recommends **Doxycycline + Rifampicin** for 6 weeks. For spondylitis, Streptomycin is often added. [2] * **Most common species:** *Brucella melitensis* (from sheep/goats) is the most virulent and common cause of human disease. [2]

Question 119: Which of the following is NOT true about fluorescent antibody detection tests in the diagnosis of Plasmodium falciparum?

A. It is an immunochromatographic test.
B. Detection of histidine-rich protein 1. (Correct Answer)
C. Detection of lactate dehydrogenase antigen.
D. Detection of glutamate dehydrogenase antigen.

Explanation: ### Explanation The question asks for the statement that is **NOT true** regarding fluorescent antibody detection or rapid diagnostic tests (RDTs) for *Plasmodium falciparum*. **1. Why "Detection of histidine-rich protein 1" is the correct answer:** The primary antigen used for the detection of *P. falciparum* in immunochromatographic tests is **Histidine-Rich Protein 2 (HRP-2)**, not HRP-1 [1]. HRP-2 is a water-soluble protein secreted by the parasite into the host's bloodstream. While HRP-1 (KAHRP) exists, it is not the target antigen for standard diagnostic RDTs. Therefore, Option B is factually incorrect. **2. Analysis of other options:** * **Option A (Immunochromatographic test):** This is true. Most rapid detection tests for malaria (including those using fluorescence) utilize immunochromatography (lateral flow) to capture parasite-specific antigens [1]. * **Option C (Lactate dehydrogenase - pLDH):** This is true. pLDH is a functional enzyme produced by living parasites [1]. It is used to detect both *P. falciparum* and non-falciparum species (*P. vivax, P. ovale, P. malariae*). * **Option D (Glutamate dehydrogenase - pGDH):** This is true. pGDH is another metabolic enzyme used as a biomarker in various rapid diagnostic platforms to identify the presence of Plasmodium species. ### High-Yield Clinical Pearls for NEET-PG: * **HRP-2 Advantage:** It is highly sensitive for *P. falciparum* [1]. However, it can remain positive for 2–4 weeks even after successful treatment (delayed clearance) [1]. * **pLDH Advantage:** It correlates well with viable parasites; thus, it becomes negative quickly after effective treatment, making it useful for monitoring drug efficacy. * **Prozone Effect:** Very high parasitemia can sometimes lead to false-negative results in HRP-2 RDTs. * **Gold Standard:** Despite the convenience of RDTs, **Peripheral Blood Smear (Thin and Thick)** remains the gold standard for malaria diagnosis.

Question 120: What is the most common diagnostic test for HIV infection?

A. ELISA (Correct Answer)
B. Western blot
C. Northern blot
D. Virus isolation

Explanation: **Explanation:** The diagnosis of HIV infection primarily relies on the detection of antibodies against the virus. **ELISA (Enzyme-Linked Immunosorbent Assay)** is the most common diagnostic test because it is highly sensitive, cost-effective, and suitable for high-throughput screening [1]. In clinical practice, the 4th generation ELISA is the standard of care as it detects both the **p24 antigen** (which appears early) and **HIV-1/2 antibodies**, significantly narrowing the "window period." **Analysis of Options:** * **B. Western Blot:** Historically, this was the "gold standard" confirmatory test due to its high specificity. However, it is complex, expensive, and takes longer to perform [1]. Current CDC/WHO guidelines have largely replaced it with rapid confirmatory immunoassays or nucleic acid testing (NAT). * **C. Northern Blot:** This is a laboratory technique used to detect specific **RNA** sequences. While HIV is an RNA virus, Northern Blotting is used in research settings and is not a clinical diagnostic tool for HIV. * **D. Virus Isolation:** Culturing HIV from blood or tissues is technically demanding, time-consuming, and hazardous. It is reserved for research purposes and is never used for routine diagnosis. **High-Yield Clinical Pearls for NEET-PG:** * **Screening Test:** ELISA (High sensitivity) [1]. * **Confirmatory Test:** Western Blot (High specificity) or Geenius™ HIV 1/2 Supplemental Assay [1]. * **Early Diagnosis (Window Period):** **p24 antigen** detection or **HIV-RNA PCR** (the earliest marker to become positive, usually within 10–12 days). * **Diagnosis in Infants (<18 months):** Antibody tests are unreliable due to persistent maternal IgG. The investigation of choice is **DNA PCR**. * **Monitoring Treatment:** Viral load (HIV-RNA) is used to monitor ART efficacy, while CD4 count monitors immune status.

Practice by Chapter

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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