Infectious Diseases — MCQs

A 54-year-old diabetic patient presents with an unresolved skin lesion on his foot. The lesion began several weeks ago as a blister and has since become a painful, erosive, expanding sore. On examination, the affected site is now 5 cm in diameter, with a black necrotic center and raised red edges. Which of the following toxins has a mechanism of action most similar to the toxin responsible for tissue damage in this patient?

Q76Medium

A patient diagnosed with TB meningitis is started on antituberculosis treatment. Facial weakness persists despite treatment. The physician administers high-dose steroids, and within one week, the patient develops pain radiating down the back of the right leg and difficulty dorsiflexing the right foot. What is the most likely cause of these new symptoms?

Q77Medium

A patient with HIV disease is on Highly Active Anti-Retroviral Therapy (HAART). The patient develops tuberculosis. Which one of the following anti-tubercular drugs interferes with HAART?

Q78Medium

A 56-year-old man presents with painful rashes over his right upper eyelid and forehead for the last 48 hours. He underwent chemotherapy for Non-Hodgkin's lymphoma one year ago. His vital signs are stable. Physical examination reveals no other abnormalities. What is the most likely diagnosis?

Q79Easy

Which of the following is not typically seen in Weil's disease?

Q80Easy

A patient presents with headache, fever, and stiff neck. What is the most likely organism responsible?

Infectious Diseases Indian Medical PG Practice Questions and MCQs

Question 71: Which of the following is true about Lyme disease?

A. Arthritis is an early manifestation.
B. Conduction block is not seen.
C. Mood disorder is a chronic neurological manifestation. (Correct Answer)
D. The causative organism is Borrelia recurrentis.

Explanation: **Explanation:** Lyme disease, caused by the spirochete **Borrelia burgdorferi**, progresses through three distinct stages [1]. Understanding the chronicity of symptoms is key to answering this question. **1. Why Option C is Correct:** Chronic neurological manifestations (Stage 3 or Late Disseminated Disease) occur months to years after the initial infection. These include **Lyme encephalopathy**, which typically presents as subtle cognitive impairments (memory loss, word-finding difficulties) and **mood disorders** (depression, irritability). Other late features include chronic axonal polyneuropathy. **2. Why the Other Options are Incorrect:** * **Option A:** Arthritis is a **late manifestation** (Stage 3), typically presenting as intermittent or persistent swelling in large joints, most commonly the knee. * **Option B:** Conduction block (specifically **AV block**) is a classic feature of early disseminated disease (Stage 2). It is the most common cardiac manifestation and can range from first-degree to complete heart block. * **Option C:** The causative organism is **Borrelia burgdorferi** (transmitted by *Ixodes* ticks) [1]. *Borrelia recurrentis* is the agent responsible for louse-borne Relapsing Fever. **High-Yield Clinical Pearls for NEET-PG:** * **Stage 1 (Early Localized):** Pathognomonic **Erythema Chronicum Migrans** (Bull’s eye rash) [1]. * **Stage 2 (Early Disseminated):** Multiple secondary annular lesions, **Bilateral Bell’s Palsy** (highly suggestive), and AV conduction blocks. * **Treatment:** **Doxycycline** is the drug of choice for early stages. For neurological (except isolated Bell's palsy) or severe cardiac involvement, **IV Ceftriaxone** is preferred. * **Vector:** *Ixodes* ticks [1].

Question 72: All of the following are true regarding Bedaquiline use except?

A. The patient must be >= 18 years with MDR TB.
B. Patients with XDR-TB are eligible for its therapy.
C. Patients should be using hormonal based birth control measures throughout the treatment period. (Correct Answer)
D. Pregnant patients are excluded.

Explanation: Bedaquiline is a diarylquinoline that inhibits mycobacterial ATP synthase, specifically used in the treatment of Multidrug-Resistant (MDR) and Extensively Drug-Resistant (XDR) Tuberculosis. **Why Option C is the correct answer (False statement):** Bedaquiline is primarily metabolized by the **CYP3A4 enzyme**. Many hormonal contraceptives are also metabolized by this pathway or can be affected by concomitant anti-TB drugs like Rifamycins (though Rifampicin is usually avoided with Bedaquiline due to significant induction). More importantly, the WHO and national guidelines recommend **non-hormonal or barrier methods** of contraception during Bedaquiline therapy to avoid potential drug-drug interactions and to ensure maximum efficacy in preventing pregnancy, as the safety profile in pregnancy is not fully established. **Analysis of other options:** * **Option A & B:** Bedaquiline is indicated for pulmonary MDR-TB and XDR-TB in adults (≥18 years) when an effective treatment regimen cannot otherwise be provided. (Note: Recent updates have lowered the age limit in some guidelines, but for standard NEET-PG questions, 18 remains the traditional benchmark). * **Option D:** Due to a lack of adequate and well-controlled studies in humans, Bedaquiline is generally **excluded/avoided in pregnant women** unless the benefits clearly outweigh the risks. **Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Inhibits the proton pump of **ATP synthase**. * **Major Side Effect:** **QTc prolongation**. Baseline and periodic ECGs are mandatory. * **Contraindication:** Avoid drugs that prolong QT interval (e.g., Moxifloxacin, Clarithromycin) and potent CYP3A4 inhibitors/inducers. * **Half-life:** Extremely long (approx. 5.5 months) due to extensive tissue distribution.

Question 73: Oral hairy leukoplakia is seen in which of the following conditions?

A. AIDS (Correct Answer)
B. Hepatitis B
C. Smoker's keratitis
D. Candidiasis

Explanation: **Oral Hairy Leukoplakia (OHL)** is a white, corrugated (hair-like) lesion typically found on the lateral borders of the tongue. It is caused by the opportunistic reactivation of the **Epstein-Barr Virus (EBV)** in the setting of profound secondary immunodeficiency. 1. **Why AIDS is correct:** OHL is one of the most specific clinical indicators of **HIV infection** [1]. It occurs when the CD4+ T-cell count drops (usually below 200-300 cells/mm³). Unlike other white lesions, OHL is non-adherent and cannot be scraped off. 2. **Why other options are incorrect:** * **Hepatitis B:** This is a systemic viral infection affecting the liver; it does not typically present with opportunistic oral mucosal lesions like OHL. * **Smoker’s Keratitis (Leukoplakia):** While smoking causes oral leukoplakia, these are precancerous lesions related to chemical irritation, not viral reactivation. They are generally flat and can occur anywhere in the mouth. * **Candidiasis (Oral Thrush):** This is a fungal infection [1]. The key clinical distinction is that Candidal plaques **can be scraped off**, leaving an erythematous (bloody) base, whereas OHL cannot. **High-Yield Clinical Pearls for NEET-PG:** * **Etiology:** EBV (Human Herpesvirus 4). * **Pathology:** Characterized by acanthosis, hyperkeratosis, and "ballooning cells" in the upper stratum spinosum. * **Significance:** Its presence in an undiagnosed patient is a strong indication to screen for HIV. * **Treatment:** Usually not required as it is asymptomatic, but it responds to HAART (Highly Active Antiretroviral Therapy) as the immune system recovers.

Question 74: Which of the following is a predisposing factor for pneumococcal pneumonia?

A. Chronic renal failure (CRF)
B. Lymphoma
C. Old age
D. All of the above (Correct Answer)

Explanation: **Explanation:** *Streptococcus pneumoniae* (Pneumococcus) is the most common cause of community-acquired pneumonia (CAP) [1]. Its pathogenicity is primarily determined by its polysaccharide capsule, which prevents phagocytosis. Any condition that impairs the immune system’s ability to clear encapsulated bacteria increases the risk of infection. **Why "All of the Above" is correct:** * **Chronic Renal Failure (CRF):** Patients with CRF exhibit "uremic immunosuppression." This involves impaired B and T-cell function, defective chemotaxis, and reduced complement activity, making them highly susceptible to pyogenic infections like Pneumococcus. * **Lymphoma:** Hematologic malignancies, especially lymphomas and Chronic Lymphocytic Leukemia (CLL), lead to hypogammaglobulinemia (low antibody levels). Since opsonizing antibodies are essential for clearing encapsulated organisms, these patients are at high risk. * **Old Age:** Immunosenescence (the gradual deterioration of the immune system with age) and a higher prevalence of comorbidities (COPD, heart failure) significantly increase the risk and severity of pneumococcal disease in the elderly [1]. **Other Predisposing Factors:** * **Splenectomy/Asplenia:** The most critical risk factor, as the spleen is the primary site for filtering encapsulated bacteria. * **Chronic Diseases:** COPD, Diabetes Mellitus, and Alcoholism (which impairs the cough reflex and ciliary action) [1]. * **HIV Infection:** Increases the risk of invasive pneumococcal disease (IPD) by almost 100-fold [1]. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Presentation:** Sudden onset of high-grade fever, productive cough with **"rusty sputum,"** and signs of lobar consolidation [2]. * **Vaccination:** The CDC recommends the **PCV15/PCV20** or **PPSV23** vaccines for high-risk groups (elderly, asplenic, or immunocompromised). * **Quellung Reaction:** A gold-standard laboratory test where the capsule swells when exposed to specific antisera.

Question 75: A 54-year-old diabetic patient presents with an unresolved skin lesion on his foot. The lesion began several weeks ago as a blister and has since become a painful, erosive, expanding sore. On examination, the affected site is now 5 cm in diameter, with a black necrotic center and raised red edges. Which of the following toxins has a mechanism of action most similar to the toxin responsible for tissue damage in this patient?

A. Anthrax toxin
B. Botulinum toxin
C. Cholera toxin
D. Diphtheria toxin (Correct Answer)

Explanation: ### Explanation **Diagnosis:** The clinical presentation describes **Ecthyma gangrenosum**, a characteristic skin lesion caused by ***Pseudomonas aeruginosa***, typically seen in immunocompromised or diabetic patients [2]. It starts as an erythematous macule that evolves into a hemorrhagic bulla and eventually a necrotic ulcer with a black eschar and surrounding erythema. **Mechanism of Action:** The primary virulence factor responsible for tissue necrosis in *Pseudomonas aeruginosa* is **Exotoxin A**. Its mechanism of action is identical to **Diphtheria toxin** (*Corynebacterium diphtheriae*). Both toxins act as ADP-ribosyltransferases [1]. These enzymes catalyze the ADP-ribosylation of the unique amino acid diphthamide in mammalian cells, which inactivates **Elongation Factor-2 (EF-2)** [1]. This inhibition halts protein synthesis, leading to cell death and subsequent tissue necrosis. **Analysis of Incorrect Options:** * **A. Anthrax toxin:** Produced by *B. anthracis*, it consists of Edema Factor (increases cAMP) and Lethal Factor (a zinc metalloproteinase that cleaves MAP kinase). * **B. Botulinum toxin:** Produced by *C. botulinum*, it acts as a protease that cleaves SNARE proteins, preventing the release of acetylcholine at the neuromuscular junction. * **C. Cholera toxin:** Produced by *V. cholerae*, it permanently activates Gs alpha subunits, leading to constitutive activation of adenylate cyclase and increased cAMP levels in intestinal cells. **NEET-PG High-Yield Pearls:** * **Exotoxin A (Pseudomonas) = Diphtheria Toxin:** Both inhibit EF-2 via ADP-ribosylation [1]. * **Ecthyma gangrenosum** is a sign of *Pseudomonas* bacteremia; it is **not** caused by *Staph aureus* (which causes Ecthyma). * *Pseudomonas* is a Gram-negative, aerobic, non-lactose fermenting motile bacillus (oxidase positive). * Other toxins that increase cAMP: *V. cholerae*, *E. coli* (LT), *B. anthracis* (EF), and *B. pertussis*.

Question 76: A patient diagnosed with TB meningitis is started on antituberculosis treatment. Facial weakness persists despite treatment. The physician administers high-dose steroids, and within one week, the patient develops pain radiating down the back of the right leg and difficulty dorsiflexing the right foot. What is the most likely cause of these new symptoms?

A. Borrelia radiculopathy (Correct Answer)
B. Diabetic mononeuritis multiplex
C. Isoniazid neuropathy
D. Tuberculosis radiculopathy

Explanation: **Explanation:** The correct answer is **Borrelia radiculopathy (Lyme disease)**. This case illustrates a classic clinical pitfall: **misdiagnosis of Lyme disease as Tuberculosis (TB)**. Both conditions can present with lymphocytic pleocytosis in CSF and cranial nerve palsies (especially facial nerve palsy). 1. **Why it is correct:** The patient likely had Lyme disease initially, presenting with facial palsy (Stage 2 disseminated Lyme) [1]. When high-dose steroids were administered without appropriate antibiotics (like Ceftriaxone or Doxycycline), the *Borrelia burgdorferi* infection was exacerbated due to immunosuppression. This led to **Bannwarth Syndrome**, characterized by the triad of lymphocytic meningitis, cranial neuritis, and painful radiculoneuritis (the "pain radiating down the leg" and foot drop). 2. **Why incorrect options are wrong:** * **Tuberculosis radiculopathy:** While TB can cause spinal involvement (Pott’s disease or arachnoiditis), it typically presents chronically as a discitis spreading along ligaments [2]. The rapid onset of radicular pain immediately following steroid administration is more characteristic of an opportunistic flare of a spirochetal infection. * **Isoniazid (INH) neuropathy:** This typically presents as a symmetric, distal "glove and stocking" sensory polyneuropathy due to Vitamin B6 deficiency, not an acute, asymmetrical painful radiculopathy. * **Diabetic mononeuritis multiplex:** While it causes asymmetrical nerve deficits, there is no history of diabetes provided, and the temporal relationship with steroid use for "TB" points toward an infectious etiology. **Clinical Pearls for NEET-PG:** * **Bannwarth Syndrome:** A hallmark of European Lyme disease; look for the triad of meningitis, cranial nerve palsy (often bilateral VII), and radiculopathy [1]. * **Steroid Caution:** Administering steroids in an undiagnosed infection can "unmask" or worsen the underlying pathogen. * **CSF Mimicry:** Both TB and Lyme show low glucose, high protein, and lymphocytic pleocytosis [2]. Always consider Lyme in the differential of "atypical" TB meningitis.

Question 77: A patient with HIV disease is on Highly Active Anti-Retroviral Therapy (HAART). The patient develops tuberculosis. Which one of the following anti-tubercular drugs interferes with HAART?

A. Isoniazid
B. Ethambutol
C. Pyrazinamide
D. Rifampicin (Correct Answer)

Explanation: **Explanation:** The correct answer is **Rifampicin**. **1. Why Rifampicin is correct:** Rifampicin is a potent **inducer of the Cytochrome P450 (CYP450) enzyme system**, specifically the CYP3A4 isoenzyme. Most Protease Inhibitors (PIs) and Non-Nucleoside Reverse Transcriptase Inhibitors (NNRTIs) used in HAART are substrates of this same enzyme system. By inducing these enzymes, Rifampicin significantly increases the metabolism of antiretroviral drugs, leading to sub-therapeutic plasma levels, treatment failure, and the development of drug-resistant HIV strains. **2. Why other options are incorrect:** * **Isoniazid (A):** While Isoniazid is a CYP450 inhibitor, it does not have the profound inducing effect that compromises HAART efficacy [1]. Its primary interaction concern is peripheral neuropathy, which can be additive if used with Stavudine (d4T). * **Ethambutol (B) and Pyrazinamide (C):** These drugs do not significantly induce or inhibit the hepatic microsomal enzyme system and do not have major pharmacokinetic interactions with antiretroviral agents. **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Rifabutin:** In patients on HAART (especially those on PIs), **Rifabutin** is the preferred alternative to Rifampicin because it is a much weaker inducer of the CYP3A4 system. * **Integrase Inhibitors:** If Rifampicin must be used with Dolutegravir, the dose of Dolutegravir must be increased (doubled to 50mg BD). * **IRIS:** Be alert for Immune Reconstitution Inflammatory Syndrome (IRIS) when starting HAART in a TB patient; typically, TB treatment is started first, followed by HAART within 2–8 weeks [2]. * **Mnemonic:** Rifampicin "Ramps up" enzymes (Inducer); Isoniazid "Inhibits" enzymes.

Question 78: A 56-year-old man presents with painful rashes over his right upper eyelid and forehead for the last 48 hours. He underwent chemotherapy for Non-Hodgkin's lymphoma one year ago. His vital signs are stable. Physical examination reveals no other abnormalities. What is the most likely diagnosis?

A. Impetigo
B. Herpes zoster (Correct Answer)
C. Pyoderma gangrenosum
D. Erysipelas

Explanation: ### Explanation **Correct Answer: B. Herpes zoster** **Clinical Reasoning:** The presentation of painful, unilateral rashes localized to a specific dermatome (in this case, the **ophthalmic division of the Trigeminal nerve - V1**) is classic for **Herpes Zoster (Shingles)** [1]. The patient’s history of chemotherapy for Non-Hodgkin’s lymphoma indicates an immunocompromised state, which is a major risk factor for the reactivation of the latent Varicella-Zoster Virus (VZV) from the sensory ganglia [1]. When V1 is involved, it is termed *Herpes Zoster Ophthalmicus*. **Analysis of Incorrect Options:** * **A. Impetigo:** Usually presents as honey-colored crusting, typically in children. It is not dermatomal and is generally painless or mildly pruritic rather than severely painful. * **C. Pyoderma gangrenosum:** An inflammatory neutrophilic dermatosis presenting as rapidly enlarging, painful ulcers with undermined violaceous edges, often associated with IBD or RA, not a dermatomal vesicular rash. * **D. Erysipelas:** A superficial bacterial cellulitis (usually Group A Strep) characterized by a well-demarcated, raised, bright red "orange-peel" (peau d'orange) lesion [2]. While it can affect the face, it lacks the dermatomal distribution and vesicular nature of Zoster. **High-Yield NEET-PG Pearls:** * **Hutchinson’s Sign:** Vesicles on the tip or side of the nose indicate involvement of the nasociliary nerve (branch of V1) and predict a high risk of ocular complications (e.g., keratitis). * **Treatment:** Oral Acyclovir, Valacyclovir, or Famciclovir should ideally be started within **72 hours** of rash onset [1]. * **Complication:** The most common chronic complication is **Post-herpetic neuralgia (PHN)**, defined as pain persisting >90 days after the rash heals [1]. * **Ramsay Hunt Syndrome:** Reactivation in the geniculate ganglion involving CN VII and VIII (triad of facial palsy, ear pain, and vesicles in the auditory canal).

Question 79: Which of the following is not typically seen in Weil's disease?

A. Fever
B. Jaundice
C. Renal failure
D. Lung involvement (Correct Answer)

Explanation: **Explanation:** **Leptospirosis** is a zoonotic infection caused by *Leptospira interrogans*. It typically presents in two phases: an initial anicteric phase and a more severe icteric phase known as **Weil’s Disease** [1]. **Why "Lung involvement" is the correct answer:** While pulmonary symptoms (like Pulmonary Hemorrhage Syndrome) can occur in severe leptospirosis, they are **not** part of the classic clinical triad that defines **Weil’s Disease**. Weil’s disease is specifically characterized by the triad of **Jaundice, Renal Failure, and Hemorrhage**. In the context of standard medical examinations like NEET-PG, lung involvement is considered a separate severe manifestation rather than a defining feature of the Weil’s triad. **Analysis of Incorrect Options:** * **A. Fever:** This is the most common presenting symptom of both the septicemic and immune phases of leptospirosis [1]. * **B. Jaundice:** A hallmark of Weil’s disease. Unlike viral hepatitis, the jaundice in Weil’s is often "orange-yellow" and occurs despite minimal hepatic necrosis. * **C. Renal failure:** Acute Kidney Injury (AKI) is a defining feature, typically presenting as non-oliguric hypokalemic renal failure due to tubular damage [1]. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Triad of Weil’s:** Jaundice + Azotemia (Renal failure) + Hemorrhages (Purpura/Epistaxis). * **Conjunctival Suffusion:** Pathognomonic sign (redness of eyes without inflammatory exudate). * **Gold Standard Test:** Microscopic Agglutination Test (MAT) [1]. * **Drug of Choice:** Doxycycline (mild cases/prophylaxis) or IV Penicillin G (severe cases). * **Occupational Risk:** Farmers, sewage workers, and those exposed to rodent urine in water [1].

Question 80: A patient presents with headache, fever, and stiff neck. What is the most likely organism responsible?

A. Staphylococcus Aureus
B. Listeria
C. Streptococcus Agalactiae
D. Streptococcus Pneumoniae (Correct Answer)

Explanation: ### Explanation The clinical triad of **headache, fever, and neck stiffness (nuchal rigidity)** is the classic presentation of **Acute Bacterial Meningitis** [1]. **Why Streptococcus pneumoniae is correct:** *Streptococcus pneumoniae* (Pneumococcus) is the **most common cause** of community-acquired bacterial meningitis in adults of all ages, as well as in children over the age of 2. It is often associated with concurrent infections like pneumonia, otitis media, or sinusitis. It carries the highest mortality and morbidity rates among the common causative agents [1]. **Analysis of Incorrect Options:** * **Staphylococcus aureus (A):** This is an uncommon cause of meningitis. It is typically seen in specific clinical contexts, such as post-neurosurgical procedures, head trauma, or secondary to infective endocarditis/bacteremia [2]. * **Listeria monocytogenes (B):** This is an important cause in specific "at-risk" populations: neonates, the elderly (>65 years), pregnant women, and immunocompromised individuals. It is not the most common cause in the general population. * **Streptococcus agalactiae (Group B Strep) (C):** This is the **most common cause of meningitis in neonates** (0–3 months), transmitted during birth from the maternal genital tract. **High-Yield Clinical Pearls for NEET-PG:** * **Most common cause overall:** *S. pneumoniae*. * **Most common cause in adolescents/young adults:** *Neisseria meningitidis* (often associated with a petechial rash). * **Empiric Treatment:** Ceftriaxone + Vancomycin [1]. Add **Ampicillin** if *Listeria* is suspected (extremes of age/immunocompromised). * **Dexamethasone:** Should be administered shortly before or with the first dose of antibiotics to reduce neurological complications (especially in Pneumococcal meningitis). * **CSF Findings in Bacterial Meningitis:** High opening pressure, high PMNs (neutrophils), low glucose (<40 mg/dL), and high protein [1].

Practice by Chapter

Principles of Antimicrobial Therapy

Practice Questions

Fever of Unknown Origin

Practice Questions

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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