Okay, let's tackle this question. The patient is a 23-year-old male who had unprotected sex with a commercial sex worker. Two weeks later, he has a painless, indurated ulcer on the glans with clear serum exudate and non-tender inguinal lymphadenopathy. The key here is to identify the likely diagnosis and the appropriate test.

First, the presentation is classic for primary syphilis. Syphilis is caused by Treponema pallidum. The primary lesion is a chancre, which is painless, indurated, and has serous exudate. The lymph nodes are usually non-tender and firm. The time frame of two weeks aligns with the typical 3-week incubation period for syphilis. So the next step would be to test for syphilis. The most accurate test is dark field microscopy to visualize the organism in the exudate. Alternatively, a VDRL or RPR test could be done, but these are non-treponemal and might not be positive yet in early stages. However, dark field microscopy is more direct and can confirm the diagnosis early on.

Looking at other options, if it were a chancroid (Haemophilus ducreyi), the ulcer would be painful, and the lymph nodes would be tender and suppurative. Herpes would present with multiple painful ulcers and vesicles, and the lymphadenopathy would be tender. Lymphogranuloma venereum (LGV) is caused by Chlamydia trachomatis and presents with a painless ulcer followed by tender, suppurative lymph nodes. So the non-tender lymph nodes here point away from LGV.

The clinical pearl here is that syphilis's painless chancre with non-tender lymph nodes is a key differentiator. Remembering the classic presentation helps in choosing the right test. The best diagnostic test in this case would be dark field microscopy for T. pallidum in the exudate.

**Core Concept**
This question assesses the differential diagnosis of genital ulcers and the appropriate diagnostic approach for syphilis. A painless, indurated ulcer (chancre) with non-tender lymphadenopathy is characteristic of *Treponema pallidum* infection (syphilis). The clinical presentation aligns with the primary stage of syphilis, where early diagnosis is critical for effective treatment.

**Why the Correct Answer is Right**
The most appropriate test is **dark field microscopy** of exudate from the ulcer to visualize *Treponema pallidum*. This direct visualization confirms the presence of the spirochete and is highly specific in early syphilis. Alternatively, a nucleic acid amplification test (NAAT) or fluorescent treponemal antibody absorption (FTA-ABS) test may be used, but dark field microscopy is the gold standard for immediate diagnosis in primary syphilis. The painless nature, induration, serum exudate, and non-tender lymphadenopathy all point to syphilis, distinguishing it from other causes like chancroid or herpes.

**Why Each Wrong Option is Incorrect**
**Option A:** *Chancroid culture* is incorrect because *Haemophilus ducreyi* causes painful ulcers with tender, suppurative lymphadenopathy, not painless,

✓ Correct Answer: B. Darkfield microscopy of ulcer discharge