Differential Diagnosis for Male on PrEP with Genital Lesions and Negative STD Testing

• Single most likely diagnosis:
  • Herpes Simplex Virus (HSV) infection: This is a common cause of genital lesions, and the fact that all STD testing came back negative does not rule out HSV, as it may require specific testing such as a viral culture or PCR to diagnose.
• Other Likely diagnoses:
  • Syphilis (with a false-negative test result): Although the patient's STD testing was negative, syphilis can sometimes yield false-negative results, especially in the early stages.
  • Genital warts (caused by Human Papillomavirus, HPV): While HPV testing might not be included in standard STD panels, genital warts can cause lesions.
  • Molluscum contagiosum: A viral infection that causes small, pearly, or flesh-colored lesions.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed):
  • Lymphogranuloma venereum (LGV): A rare but serious condition caused by certain strains of Chlamydia trachomatis, which can lead to significant morbidity if not treated promptly.
  • Primary HIV infection: Although the patient is on PrEP, non-adherence or drug resistance could lead to HIV infection, which can cause genital lesions among other symptoms.
• Rare diagnoses:
  • Behçet's disease: A rare condition that can cause genital lesions, among other symptoms like oral ulcers and eye inflammation.
  • Fixed drug eruption: A rare skin condition that can cause lesions in response to certain medications.
  • Granuloma inguinale (Donovanosis): A rare bacterial infection that can cause genital lesions, more common in certain parts of the world.