Infectious Diseases Practice Questions

Q: A patient presents with a painless genital ulcer followed by painful inguinal buboes (lymphadenopathy). Which strains of Chlamydia trachomatis are responsible for this condition?

L1–L3. ***L1–L3*** - These serovars of **Chlamydia trachomatis** are the causative agents of **lymphogranuloma venereum (LGV)**, a sexually transmitted infection. - The clinical presentation involves a transient, **painless genital ulcer** followed by the classic development of **painful inguinal lymphadenopathy** (buboes). *A, B, C* - These serovars are responsible for causing **trachoma**, a chronic keratoconjunctivitis that is the leading infectious cause of blindness worldwide. - They are typically transmitted via direct or indirect contact with eye, nose, or throat secretions rather than sexual contact. *D–K* - These strains cause common **sexually transmitted infections** including **urethritis**, **cervicitis**, and **pelvic inflammatory disease (PID)**. - They are the most frequent cause of bacterial sexually transmitted infections in developed countries but do not typically present with the pronounced **lymphadenopathy** seen in LGV. *B, C, D* - This grouping combines serovars associated with **trachoma** (B, C) and those causing **urogenital infections** (D), making it clinically incorrect. - There is no specific pathological entity associated with this collective group of **Chlamydia trachomatis** serovars.

Q: A 32-year-old woman with a history of splenectomy presents to the emergency department with sudden-onset high-grade fever, vomiting, hypotension, and rapidly progressive septic shock. Her symptoms began 2 weeks after a minor febrile illness. Which organism is the most likely causative agent of this life-threatening condition?

Streptococcus pneumoniae. ***Streptococcus pneumoniae*** - Patients with **asplenia** (post-splenectomy) are at significantly increased risk for overwhelming post-splenectomy infection (OPSI) due to defective clearance of **encapsulated organisms**. - *Streptococcus pneumoniae* is the most common cause of sepsis in asplenic patients, characterized by fulminant disease progression, high mortality rates, and rapid development of **septic shock**. *Escherichia coli* - While *Escherichia coli* is a common cause of sepsis (particularly urosepsis and intra-abdominal infections), it is not the classic pathogen associated with the unique susceptibility of the **asplenic patient**. - It lacks the specific capsular polysaccharide susceptibility profile that makes *S. pneumoniae* the primary concern in individuals without a functional **spleen**. *Staphylococcus aureus* - *Staphylococcus aureus* is a leading cause of skin and soft tissue infections, endocarditis, and osteomyelitis, but it is not typically characterized as an **encapsulated organism** that targets asplenic individuals. - The clinical presentation of rapid-onset septic shock following minor illness is more classic for the **fulminant course** of a pneumococcal infection in this specific patient demographic. *Pseudomonas aeruginosa* - *Pseudomonas aeruginosa* is an important cause of sepsis especially in **immunocompromised** or hospitalized patients, but it is not the most frequent pathogen associated with asplenia. - It generally presents in settings such as **ventilator-associated pneumonia**, burn wounds, or neutropenic fever, rather than the classic OPSI syndrome triggered by circulating **encapsulated bacteria**.

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.

Question 1

A patient presents with a painless genital ulcer followed by painful inguinal buboes (lymphadenopathy). Which strains of Chlamydia trachomatis are responsible for this condition?

Accepted Answer

L1–L3

Answer

A, B, C

Answer

D–K

Answer

B, C, D

Question 2

A female presents with high grade fever, diffuse erythematous rash and hypotension. She is
currently menstruating and using tampoons. The toxin responsible acts by?

Accepted Answer

Vbeta region of the T-cell receptor and MHC class II molecule

Answer

MHC class I molecule only

Answer

CD4 receptor only

Answer

B-cell receptor and IgG

Question 3

A 32-year-old woman with a history of splenectomy presents to the emergency department with
sudden-onset high-grade fever, vomiting, hypotension, and rapidly progressive septic shock. Her
symptoms began 2 weeks after a minor febrile illness. Which organism is the most likely causative
agent of this life-threatening condition?

Accepted Answer

Streptococcus pneumoniae

Answer

Escherichia coli

Answer

Staphylococcus aureus

Answer

Pseudomonas aeruginosa

Question 4

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 5

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

Infectious Diseases — MCQs

Infectious Diseases — MCQs

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