**Core Concept**
The patient's presentation is suggestive of a sexually transmitted infection (STI), specifically a genital ulcerative disease. The described lesion is likely a chancre, which is a hallmark of primary syphilis.

**Why the Correct Answer is Right**
The correct answer involves obtaining a sample from the chancre for microscopic examination and serological testing to confirm the diagnosis of syphilis. The chancre is a painless ulcer that typically appears 3-4 weeks after exposure to the bacterium *Treponema pallidum*. A chancre is highly suggestive of primary syphilis, and a definitive diagnosis can be made by darkfield microscopy, which reveals the characteristic spirochetal organisms. Additionally, serological tests such as the Venereal Disease Research Laboratory (VDRL) or Rapid Plasma Reagin (RPR) can be used to confirm the diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Performing a Gram stain on the lesion would not be the next step in diagnosing syphilis, as *T. pallidum* cannot be visualized with Gram staining. Instead, a darkfield microscopy or a direct immunofluorescence assay would be more appropriate.
**Option B:** While a complete blood count (CBC) may be part of the patient's overall evaluation, it is not directly relevant to the diagnosis of syphilis.
**Option C:** Obtaining a urine sample would not be the next step in diagnosing syphilis, as the diagnosis is typically made through examination of the lesion itself or serological testing.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the chancre of primary syphilis is a painless ulcer, often accompanied by regional lymphadenopathy. If left untreated, syphilis can progress through various stages, including secondary, latent, and tertiary syphilis, which can lead to significant morbidity and mortality.

**Correct Answer:** C.