A young man presents to the emergency department with a maculopapular rash 2 weeks after healing of a painless genital ulcer. The most likely etiological agent is –
**Core Concept**
The patient's presentation of a maculopapular rash 2 weeks after healing of a painless genital ulcer suggests a diagnosis of secondary syphilis, a complication of syphilis infection caused by the bacterium *Treponema pallidum*.
**Why the Correct Answer is Right**
The painless genital ulcer in the initial stage of syphilis is a classic sign of primary syphilis. The maculopapular rash in secondary syphilis is caused by the immune response to the treponemal infection, leading to a widespread inflammatory reaction. The rash can appear anywhere on the body, but it often involves the palms and soles. The correct etiological agent is *Treponema pallidum*, which is transmitted through sexual contact.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because *Haemophilus ducreyi* causes chancroid, a sexually transmitted infection characterized by painful genital ulcers, not painless ulcers.
**Option B:** This option is incorrect because *Calymmatobacterium granulomatis* causes granuloma inguinale, a sexually transmitted infection characterized by beefy red ulcers, not painless ulcers.
**Option C:** This option is incorrect because *Neisseria gonorrhoeae* causes gonorrhea, a sexually transmitted infection characterized by symptoms such as dysuria and discharge, not a maculopapular rash.
**Clinical Pearl / High-Yield Fact**
The "Rule of 2s" in syphilis is a useful mnemonic to remember the classic signs and symptoms of primary and secondary syphilis: 2 weeks for the duration of the primary lesion, 2 inches in diameter for the size of the chancre, and 2 weeks for the duration of the secondary rash.
**Correct Answer: D. Treponema pallidum**