**Question:** A 43-year-old businessman visits his primary care physician complaining of a purulent penile discharge. He had unprotected sex with a woman he met at a conference 1 week ago. Gram stain of the discharge fails to reveal any organisms. Which of the following is the most likely cause of the discharge?

A. Bacterial vaginosis
B. Syphilis
C. Trichomoniasis
D. Candidiasis

**Correct Answer: D. Candidiasis**

**Core Concept:**
Candidiasis is a fungal infection caused by the overgrowth of the fungus Candida albicans or other Candida species. It is characterized by white, curdy discharge, often associated with itching and burning sensations. In this case, the patient presents with a purulent discharge, which is unlikely due to Candidiasis.

**Why the Correct Answer is Right:**
In this scenario, the Gram stain of the discharge fails to reveal any organisms. This indicates that the infection is not bacterial, eliminating options B and C. Option A, bacterial vaginosis, would typically show Gram-negative rods on Gram stain. Option D, candidiasis, remains as the most likely cause of the patient's purulent discharge.

**Why Each Wrong Option is Incorrect:**
Option A (bacterial vaginosis) is ruled out due to the Gram stain result. Option B (syphilis) is unlikely as the patient reports unprotected sex with a single partner, decreasing the likelihood of sexually transmitted infections like syphilis. Option C (trichomoniasis) is less likely as the patient has a purulent discharge, which is not a typical presentation of trichomoniasis, which typically presents with a yellowish-green discharge.

**Clinical Pearl:**
Candidiasis, also known as vaginal thrush, is a common fungal infection affecting the genital area. It is caused by the overgrowth of Candida species, especially Candida albicans. The infection may be primary or secondary to other factors such as hormonal imbalances or immunosuppression. Candidiasis can present with symptoms like white discharge, itching, and burning sensations. The key to remember is that candidiasis is typically non-purulent and non-Gram-positive, making it less likely as the cause of the patient's purulent discharge.