Primary Syphilis (Chancre)
The most likely diagnosis is primary syphilis, presenting as a painless chancre—the classic presentation of a painless penile ulcer that began as a papule in a sexually active young adult.
Clinical Reasoning
The clinical presentation is pathognomonic for primary syphilis:
- Painless ulceration is the hallmark feature that distinguishes syphilis from other genital ulcers
- Evolution from red papule to ulcer over days follows the typical progression of a syphilitic chancre
- 19-year-old sexually active male fits the demographic for sexually transmitted infections
While herpes simplex virus is statistically the most common cause of genital ulcers in the United States 1, it typically presents with painful vesicles and ulcerations. The painless nature of this lesion essentially excludes typical herpes presentation, though it's worth noting that approximately 5% of male patients with primary herpes can present without pain 2—but this is the exception, not the rule.
Diagnostic Approach
For this patient, you should immediately obtain:
- Syphilis serology (nontreponemal test: RPR or VDRL, plus treponemal test)
- Darkfield microscopy or direct fluorescent antibody test for Treponema pallidum from the ulcer base if available 1
- PCR testing for HSV from the ulcer to definitively exclude herpes 1
- HIV testing as all patients with syphilis should be tested for HIV 3
Common Pitfalls
Do not assume all painless ulcers are syphilis without testing. The differential diagnosis of painless genital ulcers includes:
- Chancroid (though typically painful and rare in the U.S.)
- Atypical herpes (5% of cases) 2
- Traumatic ulceration with secondary infection 4
- Non-infectious causes (Behçet syndrome, fixed drug eruption) 1
However, given the classic presentation and epidemiology, empiric treatment for syphilis may be warranted while awaiting confirmatory testing, particularly if follow-up is uncertain.
Treatment Recommendation
Benzathine penicillin G 2.4 million units IM as a single dose is the treatment of choice for primary syphilis 3. For penicillin-allergic patients, doxycycline 100 mg orally twice daily for 14 days is the preferred alternative 3, though compliance and close follow-up are essential with any alternative regimen.