Infectious Diseases — MCQs

An 18-year-old woman presents with diplopia, dysarthria, and dysphagia 18 hours after consuming homemade preserves. Examination reveals cranial nerve palsies affecting extraocular movements, difficulty swallowing, and voice changes. Muscle strength is 4/5 in the arms and 5/5 in the legs, with normal reflexes. What is the most likely causative organism?

Q95Easy

For initiation of Highly Active Antiretroviral Therapy (HAART), which of the following is generally evaluated?

Q96Easy

Which meningoencephalitis presents with a soap bubble appearance?

Q97Easy

Which valve is most commonly affected in subacute bacterial endocarditis (SABE)?

Q98Medium

A 28-year-old man presents with a 2-day history of painful urination and cloudy penile discharge. Physical examination reveals purulent discharge expressed from the penis. Microscopic examination of the fluid shows Gram-negative diplococci within neutrophils. Which of the following is the most appropriate treatment?

Q99Easy

What is the approximate time interval between HIV infection and the development of AIDS?

Q100Easy

What is the drug of choice for the treatment of chlamydial infection?

Infectious Diseases Indian Medical PG Practice Questions and MCQs

Question 91: Which of the following is NOT true about Salmonella hepatitis?

A. High grade fever with relative bradycardia
B. Biochemical profile suggestive of infiltrative liver disease pattern
C. Usually anicteric
D. ALT/LDH ratio will be >5 (Correct Answer)

Explanation: **Explanation:** Salmonella hepatitis is a complication of enteric fever (Typhoid) characterized by liver involvement. Understanding the biochemical markers is crucial for differentiating it from viral hepatitis. **1. Why Option D is the Correct Answer (The False Statement):** In Salmonella hepatitis, the **ALT/LDH ratio is typically <4**. This is a high-yield diagnostic marker. While ALT is elevated, the rise in LDH (Lactate Dehydrogenase) is disproportionately higher due to the multisystemic nature of the infection and potential hemolysis. In contrast, viral hepatitis usually presents with an ALT/LDH ratio >5 [2]. **2. Analysis of Incorrect Options:** * **Option A:** Enteric fever classically presents with **high-grade fever** and **Faget’s sign (relative bradycardia)**, where the pulse rate does not increase proportionately with the temperature. * **Option B:** The biochemical profile often mimics **infiltrative liver disease** rather than pure hepatocellular necrosis [2]. This includes modest elevations in transaminases (usually <10 times the upper limit) and alkaline phosphatase. * **Option C:** Most cases of Salmonella hepatitis are **anicteric** (without jaundice). Clinical jaundice occurs in only 5-10% of cases and usually signifies a more severe course or complications like cholecystitis [1]. **Clinical Pearls for NEET-PG:** * **Histology:** Look for "Typhoid nodules" (small focal areas of hepatocyte necrosis with Kupffer cell hyperplasia and mononuclear infiltration). * **Widal Test:** Usually becomes positive in the 2nd week of illness [1]. * **Treatment:** Ceftriaxone or Azithromycin are preferred due to widespread multidrug resistance (MDR) and Nalidixic acid-resistant S. typhi (NARST). * **Key Differentiator:** Unlike viral hepatitis, Salmonella hepatitis rarely leads to fulminant hepatic failure.

Question 92: Which of the following corticosteroids is NOT used in any form of tuberculosis?

A. Miliary tuberculosis
B. Meningeal tuberculosis
C. Renal tuberculosis
D. Intestinal tuberculosis (Correct Answer)

Explanation: In the management of Tuberculosis (TB), corticosteroids are used as an **adjunct** to Anti-Tubercular Therapy (ATT) to reduce the harmful effects of the host's inflammatory response, prevent fibrosis, and decrease edema in critical anatomical sites. [1] **Why Intestinal TB is the Correct Answer:** Corticosteroids are generally **avoided** in Intestinal TB. The primary concern is that steroids can mask symptoms of an acute abdomen or, more critically, increase the risk of **bowel perforation** and delay the healing of tubercular ulcers. [1] Unlike other forms of TB, there is no proven survival benefit or reduction in stricture formation with steroid use in intestinal involvement. **Analysis of Other Options:** * **Miliary TB:** Steroids are indicated in severe cases with associated respiratory failure (ARDS) or adrenal insufficiency to reduce overwhelming systemic inflammation. * **Meningeal TB:** This is a **Class I indication**. Steroids (Dexamethasone or Prednisolone) are mandatory as they reduce cerebral edema, prevent vasculitis, and decrease the formation of basal exudates, significantly improving survival and neurological outcomes. [2] * **Renal TB:** Steroids are used specifically to prevent **ureteric strictures** and subsequent hydronephrosis during the early healing phase of the disease. **High-Yield Clinical Pearls for NEET-PG:** * **Absolute Indications for Steroids in TB:** Tuberculous Meningitis (TBM) and Tuberculous Pericarditis (to prevent constrictive pericarditis). [2] * **Other Indications:** Pleural effusion (if large/symptomatic), endobronchial TB (to prevent bronchiectasis), and laryngeal TB (to prevent airway obstruction). * **Standard Regimen:** Prednisolone 1 mg/kg/day for 4–6 weeks, followed by a gradual taper.

Question 93: What is the most common cause of dysentery?

A. Shigella dysenteriae (Correct Answer)
B. Entamoeba histolytica
C. Salmonella
D. Campylobacter

Explanation: **Explanation:** **Shigella dysenteriae** is the most common cause of dysentery worldwide. Dysentery is clinically defined as diarrhea containing visible blood and mucus, often accompanied by fever and tenesmus. Shigella is highly virulent; an extremely low infectious dose (as few as 10–100 organisms) is sufficient to cause disease. It invades the colonic epithelium and produces the **Shiga toxin**, which causes cell death, mucosal ulceration, and the characteristic bloody discharge. **Analysis of Incorrect Options:** * **Entamoeba histolytica:** While it causes "Amoebic dysentery," it is less common globally than bacterial causes. It typically presents more subacutely with "anchovy sauce" liver abscesses as a potential complication. * **Salmonella:** While *Salmonella enterica* (non-typhoidal) can cause inflammatory diarrhea, it more commonly presents as gastroenteritis without gross blood. * **Campylobacter:** *Campylobacter jejuni* is a leading cause of bacterial gastroenteritis and can cause bloody stools, but it is statistically less frequent than Shigella as a cause of classic dysentery. It is also uniquely associated with post-infectious Guillain-Barré Syndrome. **High-Yield Clinical Pearls for NEET-PG:** * **Most common species:** While *S. dysenteriae* (Type 1) causes the most severe epidemics, *S. sonnei* is the most common species in developed countries, and *S. flexneri* is most common in developing nations. [1] * **Complications:** Watch for **Hemolytic Uremic Syndrome (HUS)**, especially with *S. dysenteriae* Type 1, and **leukemoid reaction** (WBC count >50,000/mm³). * **Drug of Choice:** Fluoroquinolones (like Ciprofloxacin) or Azithromycin are typically used for treatment, though resistance patterns vary. [1]

Question 94: An 18-year-old woman presents with diplopia, dysarthria, and dysphagia 18 hours after consuming homemade preserves. Examination reveals cranial nerve palsies affecting extraocular movements, difficulty swallowing, and voice changes. Muscle strength is 4/5 in the arms and 5/5 in the legs, with normal reflexes. What is the most likely causative organism?

A. Clostridium botulinum toxin (Correct Answer)
B. Staphylococcal toxin
C. Salmonellosis
D. Brucellosis

Explanation: The clinical presentation describes the classic triad of **Botulism**: symmetric descending paralysis, clear sensorium, and absence of fever [2], [3]. **1. Why Clostridium botulinum is correct:** The patient exhibits the "4 Ds" of botulism: **Diplopia, Dysarthria, Dysphagia, and Dyspnea** [2]. The toxin produced by *C. botulinum* irreversibly binds to the presynaptic nerve terminal, preventing the release of **Acetylcholine (ACh)** at the neuromuscular junction. The history of consuming **homemade preserves** (canned foods) is a classic trigger, as the anaerobic environment allows the spores to germinate and produce toxins. The paralysis is characteristically **descending** (starting with cranial nerves and moving to limbs) and **symmetric** [3]. **2. Why the other options are incorrect:** * **Staphylococcal toxin:** Causes rapid-onset (1–6 hours) nausea, vomiting, and abdominal cramps (food poisoning) but does not cause neurological deficits or paralysis [1]. * **Salmonellosis:** Typically presents with inflammatory diarrhea, fever, and abdominal pain. It does not cause descending paralysis [1]. * **Brucellosis:** A chronic granulomatous disease presenting with undulant fever, arthralgia, and hepatosplenomegaly, usually linked to unpasteurized dairy. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Proteolysis of SNARE proteins, inhibiting ACh release. * **Reflexes:** Unlike Guillain-Barré Syndrome (GBS), which features ascending paralysis and *absent* reflexes, Botulism features descending paralysis and often *preserved* reflexes initially. * **Infant Botulism:** Caused by ingestion of **honey** (spores), leading to "Floppy Baby Syndrome" [3]. * **Treatment:** Immediate administration of Equine Antitoxin (Heptavalent) and supportive respiratory care [3]. Do not wait for lab confirmation.

Question 95: For initiation of Highly Active Antiretroviral Therapy (HAART), which of the following is generally evaluated?

A. CD4 count
B. Viral load
C. Absolute lymphocyte count
D. All of the above (Correct Answer)

Explanation: **Explanation:** The initiation and monitoring of Highly Active Antiretroviral Therapy (HAART) require a comprehensive assessment of the patient’s immune status and viral replication activity. While current WHO and NACO guidelines follow the **"Test and Treat"** policy (initiating HAART regardless of CD4 count), all three parameters mentioned remain critical for baseline evaluation and clinical decision-making. * **CD4 Count (Option A):** This is the most important indicator of immune function. It determines the stage of HIV infection, the risk of opportunistic infections (OIs), and the need for prophylactic therapy (e.g., Co-trimoxazole for *Pneumocystis jirovecii*). * **Viral Load (Option B):** Quantitative HIV RNA levels measure the rate of viral replication. Baseline viral load is a strong predictor of the rate of CD4 decline and is the "gold standard" for monitoring the efficacy of HAART after initiation. * **Absolute Lymphocyte Count (ALC) (Option C):** In resource-limited settings where CD4 testing may not be available, ALC serves as a surrogate marker. An ALC <1200 cells/µL often correlates with a CD4 count <200 cells/µL, indicating advanced disease. **Why "All of the above" is correct:** A clinician must evaluate the degree of immunosuppression (CD4/ALC) and the viral activity (Viral Load) to tailor the regimen and monitor for treatment failure or Immune Reconstitution Inflammatory Syndrome (IRIS). **High-Yield Clinical Pearls for NEET-PG:** * **Goal of HAART:** To achieve an "undetectable" viral load (usually <50 copies/mL). * **Virological Failure:** Defined as a plasma viral load >1000 copies/mL on two consecutive measurements within a 3-month interval. * **First-line Regimen (NACO):** TLD regimen (Tenofovir + Lamivudine + Dolutegravir). * **Best Predictor of Progression:** Viral load is the best predictor of progression to AIDS, while CD4 count is the best indicator of immediate risk of death.

Question 96: Which meningoencephalitis presents with a soap bubble appearance?

A. Cryptococcus (Correct Answer)
B. Mucormycosis
C. Tubercular
D. HSV

Explanation: **Explanation:** **Cryptococcus neoformans** is the correct answer. The "soap bubble appearance" is a classic neuroimaging finding (MRI/CT) seen in Cryptococcal meningoencephalitis, particularly in immunocompromised patients (e.g., HIV/AIDS). This appearance is caused by the extension of the fungus from the subarachnoid space into the **Virchow-Robin (perivascular) spaces**. As the fungus produces its characteristic thick polysaccharide capsule, these spaces dilate, forming multiple small, non-enhancing cystic lesions in the basal ganglia and thalamus that resemble soap bubbles. **Analysis of Incorrect Options:** * **B. Mucormycosis:** Typically presents as rhinocerebral disease with vascular invasion leading to hemorrhagic infarction and necrosis (black eschar), not cystic "soap bubbles." * **C. Tubercular Meningitis:** Characterized by thick basal exudates, hydrocephalus, and "ring-enhancing lesions" (tuberculomas), often involving the base of the brain [1]. * **D. HSV Encephalitis:** Classically involves the **temporal lobes**, presenting with edema, hemorrhage, and necrosis [2]. It is the most common cause of sporadic fatal encephalitis. **High-Yield Clinical Pearls for NEET-PG:** * **Organism:** *Cryptococcus neoformans* (encapsulated yeast). * **Stain:** **India Ink** (shows a clear halo against a dark background). * **Antigen Test:** Latex agglutination for capsular polysaccharide (highly sensitive/specific). * **Culture:** Sabouraud Dextrose Agar (SDA). * **Treatment:** Induction with Amphotericin B + Flucytosine, followed by Fluconazole. * **CSF Finding:** Characteristically high opening pressure.

Question 97: Which valve is most commonly affected in subacute bacterial endocarditis (SABE)?

A. Pulmonary valve
B. Mitral valve (Correct Answer)
C. Tricuspid valve
D. Aortic valve

Explanation: **Explanation:** In Subacute Bacterial Endocarditis (SABE), the **Mitral valve** is the most frequently involved valve. This is because SABE typically occurs on valves with pre-existing structural abnormalities (such as Mitral Valve Prolapse or chronic rheumatic heart disease) [1]. The mitral valve experiences high hemodynamic pressure and turbulence, making it the primary site for the deposition of sterile fibrin-platelet vegetations, which are subsequently seeded by low-virulence organisms like *Streptococcus viridans* [1]. **Analysis of Options:** * **Mitral Valve (Correct):** Statistically the most common site in non-IV drug users. If the question asks for the most common valve overall, the order of frequency is: **Mitral > Aortic > Tricuspid > Pulmonary.** * **Aortic Valve:** This is the second most common valve affected. However, if both Mitral and Aortic valves are involved simultaneously, it is highly suggestive of IV drug use or specific pathogens like *Enterococcus* [1]. * **Tricuspid Valve:** This is the most common valve involved in **IV drug users (IVDU)**, often associated with *Staphylococcus aureus* [1]. It is rarely the primary site in subacute cases involving native valves. * **Pulmonary Valve:** This is the least commonly affected valve in endocarditis (less than 1% of cases). **High-Yield Clinical Pearls for NEET-PG:** * **Most common organism in SABE:** *Streptococcus viridans* (associated with dental procedures) [1]. * **Most common organism in Acute IE:** *Staphylococcus aureus* [1]. * **Most common organism in Prosthetic Valve Endocarditis (Early <1 year):** *Staphylococcus epidermidis*. * **HACEK group:** Common cause of culture-negative endocarditis. * **Duke’s Criteria:** The gold standard for diagnosis (requires 2 major, 1 major + 3 minor, or 5 minor criteria) [2].

Question 98: A 28-year-old man presents with a 2-day history of painful urination and cloudy penile discharge. Physical examination reveals purulent discharge expressed from the penis. Microscopic examination of the fluid shows Gram-negative diplococci within neutrophils. Which of the following is the most appropriate treatment?

A. Intramuscular ceftriaxone plus oral doxycycline (Correct Answer)
B. Oral penicillin G
C. Intramuscular penicillin V
D. Intramuscular ampicillin and oral penicillin V

Explanation: **Explanation:** The clinical presentation of purulent urethral discharge and the microscopic finding of **Gram-negative intracellular diplococci (GNID)** within neutrophils are pathognomonic for **Gonococcal Urethritis** caused by *Neisseria gonorrhoeae* [1]. **1. Why Option A is Correct:** Current clinical guidelines (CDC and WHO) recommend dual therapy for urethritis. **Ceftriaxone (500 mg IM single dose)** is the drug of choice for *N. gonorrhoeae* due to increasing resistance to other antibiotics [2]. **Doxycycline (100 mg BID for 7 days)** is added to empirically treat potential co-infection with *Chlamydia trachomatis*, which occurs in up to 40% of patients with gonorrhea [2]. Even if Chlamydia is not confirmed, dual therapy is the standard of care in syndromic management. **2. Why Other Options are Incorrect:** * **Options B, C, and D:** Penicillins (Penicillin G, V, and Ampicillin) are no longer used for gonorrhea. *N. gonorrhoeae* has developed high levels of resistance through **plasmid-mediated penicillinase production** (PPNG) and chromosomal mutations. These drugs would result in high treatment failure rates. **Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Culture on **Thayer-Martin medium** (Chocolate agar with antibiotics). * **Most Sensitive Test:** Nucleic Acid Amplification Test (NAAT). * **Fitz-Hugh-Curtis Syndrome:** A complication in females involving perihepatitis ("violin-string" adhesions). * **Disseminated Gonococcal Infection (DGI):** Presents with the triad of tenosynovitis, dermatitis, and polyarthralgia. * **Partner Management:** Always treat the sexual partner(s) to prevent reinfection (Expedited Partner Therapy) [2].

Question 99: What is the approximate time interval between HIV infection and the development of AIDS?

A. 7.5 years
B. 10 years (Correct Answer)
C. 12 years
D. 15 years

Explanation: The natural history of untreated HIV infection follows a predictable clinical course [1]. After the initial acute retroviral syndrome, the patient enters a period of **clinical latency**. During this phase, the virus replicates at a steady state (viral set point) while CD4+ T-cell counts gradually decline [1]. On average, it takes approximately **10 years** for an untreated HIV-infected individual to progress to AIDS (defined as a CD4 count <200 cells/µL or the presence of an AIDS-defining illness) [1]. **Analysis of Options:** * **Option B (10 years):** This is the globally accepted average duration for "typical progressors." While some individuals progress rapidly (2–3 years) and others are "long-term non-progressors," the median interval remains a decade [1]. * **Option A (7.5 years):** This is shorter than the average median. While progression can be faster in older patients or those with high initial viral loads, it is not the standard textbook average. * **Options C & D (12 and 15 years):** These durations are too long for the average untreated patient. While modern Antiretroviral Therapy (ART) can extend this interval indefinitely, the question refers to the natural history of the disease. **NEET-PG High-Yield Pearls:** * **Window Period:** The time between infection and the appearance of detectable antibodies (usually 3–12 weeks). * **Viral Set Point:** The stable level of HIV RNA in the blood after the initial peak; it is the strongest predictor of the rate of disease progression [1]. * **Indicator Disease:** Oesophageal candidiasis is one of the most common AIDS-defining illnesses [1]. * **CD4 Thresholds:** * <500: Increased risk of TB, Herpes Zoster [1]. * <200: AIDS definition; risk of *Pneumocystis jirovecii* [1]. * <50: Risk of CMV retinitis and *Mycobacterium avium* complex (MAC).

Question 100: What is the drug of choice for the treatment of chlamydial infection?

A. Ampicillin
B. Third generation cephalosporins
C. Metronidazole
D. Doxycycline (Correct Answer)

Explanation: The drug of choice for uncomplicated chlamydial infections (caused by *Chlamydia trachomatis*) is **Doxycycline**. [1] **1. Why Doxycycline is correct:** *Chlamydia trachomatis* is an obligate intracellular bacterium. Doxycycline, a tetracycline, is highly lipophilic, allowing it to penetrate host cell membranes effectively to reach the intracellular pathogen. It inhibits protein synthesis by binding to the 30S ribosomal subunit. Current CDC and WHO guidelines recommend Doxycycline (100 mg twice daily for 7 days) over Azithromycin due to superior efficacy in eradicating rectal and urogenital infections. [1] **2. Why the other options are incorrect:** * **Ampicillin & 3rd Gen Cephalosporins:** These are Beta-lactams that target the bacterial cell wall (peptidoglycan synthesis). Since *Chlamydia* lacks a classic peptidoglycan layer and resides intracellularly, these drugs are ineffective. * **Metronidazole:** This is an antiprotozoal and anaerobic antibacterial agent. It is used for *Trichomonas vaginalis* or Bacterial Vaginosis, but has no activity against *Chlamydia*. **3. High-Yield Clinical Pearls for NEET-PG:** * **Pregnancy:** Doxycycline is contraindicated in pregnancy (risk of fetal teeth discoloration/bone growth inhibition). The drug of choice for Chlamydia in pregnant women is **Azithromycin** (1g stat). * **Co-infection:** Patients with Gonorrhea are frequently co-infected with Chlamydia. Treatment often involves Ceftriaxone (for Gonorrhea) plus Doxycycline. [1] * **Lymphogranuloma Venereum (LGV):** Caused by *C. trachomatis* serotypes L1-L3; the treatment is also Doxycycline, but for a longer duration (21 days). * **Trachoma:** The WHO "SAFE" strategy uses a single dose of Azithromycin for mass drug administration.

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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