A 26-year-old HIV-negative man presents with a painful, soft, non-indurated genital ulcer with undermined edges and a purulent base on the prepuce. Inguinal lymphadenopathy is unilateral and tender. Gram stain of pus shows small Gram-negative rods arranged in chains ('school of fish' or 'shoal of fish' pattern). What is the causative organism and recommended treatment?
- A Treponema pallidum; benzathine penicillin G IM single dose
- B Klebsiella granulomatis; doxycycline 100 mg twice daily for 21 days
- C Haemophilus ducreyi; azithromycin 1 g orally single dose or ceftriaxone 250 mg IM single dose ✓
- D Chlamydia trachomatis L1-L3; doxycycline 100 mg twice daily for 21 days
Explanation
Chancroid (soft chancre) is caused by Haemophilus ducreyi, a Gram-negative facultative anaerobic bacillus. The hallmark Gram stain appearance is small coccobacilli arranged in parallel chains ('school of fish,' 'shoal of fish,' or 'railway tracks' pattern). The ulcer is painful, soft, non-indurated with ragged undermined edges and a dirty necrotic base — the opposite of syphilitic chancre. Unilateral tender fluctuant inguinal bubo (pseudo-bubo) may develop. Recommended treatment per current STI guidelines: azithromycin 1 g orally as a single dose or ceftriaxone 250 mg IM single dose. Klebsiella granulomatis causes donovanosis (granuloma inguinale).
Reference: Neena Khanna Illustrated Synopsis of Dermatology & STD, 6th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.