A 25-year-old female presents with vulvar pruritus and a thick, white vaginal discharge. What test is most helpful for diagnosis?

The most helpful test for diagnosing the condition described in the scenario, which includes vulvar pruritus and a thick, white vaginal discharge, is a KOH prep. This test is specifically used to identify infections caused by fungi, particularly Candida species, which are often responsible for the symptoms described.

When a KOH prep is performed, a sample of vaginal discharge is mixed with potassium hydroxide (KOH), which helps clear away cellular debris and enhances the visualization of fungal elements, such as hyphae or budding yeast cells, under a microscope. In cases of candidiasis, the presence of these elements can confirm the diagnosis.

Other tests mentioned, such as a Gram stain, are useful for diagnosing bacterial infections and can differentiate between bacterial vaginosis and other types of infections, but they are less specific for fungal infections. A Tzanck smear is typically used to identify herpes infections by looking for multinucleated giant cells, which does not apply here. The FTA-ABS is a test for syphilis and is unrelated to vaginal discharge or vulvar itching. Hence, for the symptoms presented, a KOH prep is the most appropriate diagnostic tool.