A 28-year-old man returns from Southeast Asia with a painful genital ulcer with undermined edges and tender suppurative inguinal lymphadenitis. Gram stain of ulcer exudate shows gram-negative coccobacilli arranged in parallel chains ('school of fish'). The causative organism is best cultured on which medium?

• A Thayer-Martin agar
• B Chocolate agar enriched with IsoVitalex in 5% CO2 ✓
• C MacConkey agar
• D Lowenstein-Jensen medium

Explanation

Haemophilus ducreyi, the agent of chancroid, requires X factor and enriched chocolate agar with IsoVitalex in 5% CO2 for growth; the school-of-fish arrangement on smear is characteristic. Thayer-Martin is a selective medium for Neisseria and does not support H. ducreyi well, which kills the closest distractor. MacConkey supports enterics and LJ supports mycobacteria.

Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.