A 25 year-old female presents with vulvar pruritus and a thick, white vaginal discharge. Which test is most helpful in making the correct diagnosis?

The most helpful test for diagnosing the situation described is a KOH (potassium hydroxide) prep. This test is particularly effective in cases of vulvar pruritus with thick, white vaginal discharge, which are common symptoms of a vaginal yeast infection, such as candidiasis.

When a KOH prep is performed, the potassium hydroxide dissolves cellular material and yeast, allowing for clearer visualization of fungal elements under the microscope. This is crucial for identifying the presence of yeast cells or hyphae that indicate a candidal infection.

Other testing options do not provide the same specificity for diagnosing a yeast infection. A Gram stain primarily identifies bacterial organisms and is not well-suited for detecting yeast. A Tzanck smear is used for diagnosing herpes simplex virus infections by identifying multinucleated giant cells, while the FTA-ABS test is utilized for detecting syphilis and does not relate to the signs of vulvar pruritus and white discharge. Therefore, the KOH prep stands out as the most appropriate and effective diagnostic tool in this clinical scenario.