Infectious Diseases Practice Questions

Q: A 65-year-old diabetic male presents with high-grade fever, productive rust-coloured sputum, and right-sided pleuritic chest pain for 3 days. His oxygen saturation is 88% on room air. A sputum Gram stain is performed (Image 1). Which of the following tests would most definitively confirm the identity of this organism from a positive blood culture?

A. Optochin sensitivity and bile solubility. ***A. Optochin sensitivity and bile solubility*** - The clinical presentation of **rust-colored sputum** and Gram stain showing **lancet-shaped diplococci** confirms *Streptococcus pneumoniae*. - This organism is definitively identified by its **sensitivity to optochin** (Ethylhydrocupreine) and its **solubility in bile** salts, which distinguishes it from other alpha-hemolytic streptococci. *B. CAMP test and hippurate hydrolysis* - These tests are used for the identification of **Group B Streptococcus** (*Streptococcus agalactiae*), not pneumococcus. - *S. agalactiae* typically causes infections in **neonates** or postpartum women, rather than lobar pneumonia with rust-colored sputum. *C. Coagulase test and catalase test* - The **catalase test** is the primary step to differentiate *Staphylococci* (positive) from *Streptococci* (negative). - The **coagulase test** specifically identifies *Staphylococcus aureus*, which would appear as **clusters** on Gram stain rather than pairs. *D. Oxidase test and motility in SIM medium* - These tests are generally used for identifying **Gram-negative bacilli** like *Pseudomonas aeruginosa* or *Enterobacteriaceae*. - *Streptococcus pneumoniae* is a **Gram-positive** non-motile coccus and would not be evaluated using these biochemical parameters.

Q: Which of the following causes mesenteric lymphadenitis?

All of the above. **Explanation:** Mesenteric lymphadenitis is the inflammation of the lymph nodes within the mesentery, often clinically mimicking acute appendicitis (Pseudo-appendicitis syndrome). This condition is caused by a variety of infectious agents, including viral, bacterial, and parasitic pathogens. * **Yersinia pseudotuberculosis (and Y. enterocolitica):** These are the most classic bacterial causes. They have a tropism for lymphoid tissue and frequently cause terminal ileitis and mesenteric lymphadenitis, leading to right lower quadrant pain. * **Campylobacter jejuni:** While primarily known for causing inflammatory diarrhea, *Campylobacter* can also involve the mesenteric lymph nodes, presenting with a clinical picture indistinguishable from appendicitis. * **Adenovirus:** This is the most common **viral** cause of mesenteric lymphadenitis, particularly in children. It is often associated with a concurrent or recent upper respiratory tract infection. Since all three agents listed are recognized causes of this clinical syndrome, **Option D (All of the above)** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Pseudo-appendicitis:** Always consider *Yersinia enterocolitica* or *Yersinia pseudotuberculosis* in a patient presenting with appendicitis-like symptoms but a normal appendix on imaging/surgery. * **Diagnosis:** Ultrasound is the preferred initial imaging modality, showing enlarged lymph nodes (>8mm in short-axis diameter) with a normal-looking appendix. * **Other Causes:** *Salmonella typhi*, *Mycobacterium tuberculosis* (Tabes mesenterica), and Giardiasis can also lead to mesenteric nodal enlargement. * **Key Distinction:** Unlike acute appendicitis, mesenteric lymphadenitis is usually self-limiting and managed conservatively.

Q: A 45-year-old man with AIDS presents with productive cough, fever, and night sweats. Chest X-ray reveals an ill-defined area of consolidation at the periphery of the right middle lobe and mediastinal lymphadenopathy. Sputum culture yields acid-fast bacilli. The patient subsequently develops severe headache and neck rigidity. Which of the following brain areas is most likely affected by this patient's infection?

Base of the brain. **Explanation:** The clinical presentation of productive cough, fever, night sweats, and acid-fast bacilli in sputum in an immunocompromised patient (AIDS) strongly suggests **Tuberculosis (TB)**. The subsequent development of headache and neck rigidity indicates **Tuberculous Meningitis (TBM)**, a common extrapulmonary complication in HIV-infected individuals. **1. Why "Base of the brain" is correct:** Tuberculous meningitis is characterized by a thick, gelatinous **exudate** that preferentially accumulates in the **basal cisterns** (interpeduncular fossa, pontine cistern, and around the optic chiasm). This basal distribution occurs because the bacilli spread via the subarachnoid space after the rupture of a subependymal tubercle (Rich focus). This predilection for the base of the brain often leads to cranial nerve palsies (CN III, IV, and VI) and obstructive hydrocephalus. **2. Why other options are incorrect:** * **Cerebellum:** While tuberculomas can occur here, it is not the primary site for the characteristic meningeal exudate. * **Hippocampus:** Classically associated with **Herpes Simplex Virus (HSV-1) encephalitis** or Rabies (Negri bodies), not TB. * **Periventricular white matter:** This is the characteristic site for **Progressive Multifocal Leukoencephalopathy (PML)** caused by the JC virus or Primary CNS Lymphoma in AIDS patients. **Clinical Pearls for NEET-PG:** * **Rich Focus:** A small tuberculous lesion in the brain parenchyma or meninges that ruptures into the subarachnoid space to cause TBM. * **CSF Findings in TBM:** High protein, very low glucose, and **lymphocytic pleocytosis** (Cobweb coagulum formation). * **Imaging:** Contrast-enhanced CT/MRI typically shows **basal meningeal enhancement**. * **Complication:** The most common cranial nerve involved in TBM is the **6th Nerve (Abducens)**.

Question 1

A 34-year-old man with a CD4 count of 48 cells/µL presents with a 3-week history of progressive headache, low-grade fever, and neck stiffness. Lumbar puncture reveals elevated opening pressure, low glucose, and elevated protein. CSF microscopy is shown (Image 1). Which of the following best describes the virulence mechanism of the organism identified that directly enables its evasion of phagocytic killing in the host?

Accepted Answer

A. Production of a polysaccharide capsule that inhibits complement-mediated opsonisation and phagocytosis

Answer

B. Formation of a heat-stable lipopolysaccharide that activates TLR-4 and suppresses macrophage oxidative burst

Answer

C. Secretion of a pore-forming exotoxin that lyses neutrophils before phagocytic engulfment occurs

Answer

D. Expression of protein A on the cell wall that binds the Fc region of IgG and prevents opsonin recognition

Question 2

A 65-year-old diabetic male presents with high-grade fever, productive rust-coloured sputum, and right-sided pleuritic chest pain for 3 days. His oxygen saturation is 88% on room air. A sputum Gram stain is performed (Image 1). Which of the following tests would most definitively confirm the identity of this organism from a positive blood culture?

Accepted Answer

A. Optochin sensitivity and bile solubility

Answer

B. CAMP test and hippurate hydrolysis

Answer

C. Coagulase test and catalase test

Answer

D. Oxidase test and motility in SIM medium

Question 3

Which of the following is NOT implicated in congenital transmission?

Accepted Answer

Hepatitis A

Answer

Toxoplasmosis

Answer

Herpes

Answer

Syphilis

Question 4

Which of the following causes mesenteric lymphadenitis?

Accepted Answer

All of the above

Answer

Adenovirus

Answer

Campylobacter

Answer

Yersinia pseudotuberculosis

Question 5

A 45-year-old man with AIDS presents with productive cough, fever, and night sweats. Chest X-ray reveals an ill-defined area of consolidation at the periphery of the right middle lobe and mediastinal lymphadenopathy. Sputum culture yields acid-fast bacilli. The patient subsequently develops severe headache and neck rigidity. Which of the following brain areas is most likely affected by this patient's infection?

Accepted Answer

Base of the brain

Answer

Cerebellum

Answer

Hippocampus

Answer

Periventricular white matter

Infectious Diseases — MCQs

Infectious Diseases — MCQs

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