Trichomoniasis

Trichomoniasis is a sexually transmitted infection caused by Trichomonas vaginalis, a flagellated protozoan — not a bacterium, not a fungus. That distinction matters because USMLE Step 1 will test whether you know how to diagnose it (wet mount, not Gram stain) and how to treat it (metronidazole, with a specific counseling point about alcohol). In women, the classic presentation is a frothy, yellow-green vaginal discharge with vulvar pruritus and a 'strawberry cervix' (punctate hemorrhages on the cervix). Men are often asymptomatic carriers, which the exam may use to set up a clinical vignette where a woman keeps getting reinfected.

The exam hits this concept from two angles: recognizing the clinical presentation and choosing the right diagnostic test, and then managing the patient correctly including partner treatment. For diagnosis questions, the stem usually describes discharge characteristics and asks what you'd see on microscopy — the answer is motile flagellated protozoa on wet mount, with NAAT as the gold standard in clinical practice. Step 1 may test this by including a distractor answer about Gram stain (which is for bacterial infections like gonorrhea or BV).

The trickiest part isn't recognizing Trichomonas — it's the metronidazole-alcohol interaction. Students often know metronidazole is the drug but forget the disulfiram-like reaction. If a vignette asks about counseling or a patient develops flushing and vomiting after a glass of wine while on metronidazole, that's your cue. Both partners must be treated simultaneously, or reinfection is guaranteed — another classic Step 1 trap.