Chancroid is most commonly caused by which organism?

• A. Treponema pallidum
• B. Staph aureus
• C. Neisseria gonorrhoeae
• D. Haemophilus ducreyi

Answer: (D)

Chancroid is caused by Haemophilus ducreyi, a fastidious gram-negative coccobacillus. This organism produces painful genital ulcers with ragged edges and typically painful, tender inguinal lymphadenopathy. Because H. ducreyi grows only on specialized culture media, clinicians often rely on clinical presentation and, when available, specific testing to support the diagnosis.

This differs from other organisms in key ways: Treponema pallidum causes a painless chancre of syphilis, not a painful ulcer; Neisseria gonorrhoeae mainly causes urethral discharge and gonorrheal infections rather than a painful genital ulcer with tender nodes; Staphylococcus aureus is associated with skin infections like abscesses rather than the characteristic chancroid ulcer. Thus Haemophilus ducreyi best fits the presentation of chancroid.