A 26-year-old female presents with pruritic lesions consistent with contact dermatitis. What is the most useful diagnostic test?

The most useful diagnostic test for a patient presenting with pruritic lesions consistent with contact dermatitis is patch testing. This test is specifically designed to identify allergens that may be causing the dermatitis by exposing the skin to small amounts of various substances and observing for any reactions. Patch testing can be particularly helpful in determining the presence of delayed-type hypersensitivity reactions that are typical of contact dermatitis, where the immune system reacts to allergens after a period of exposure.

This method allows for the identification of specific allergens, such as metals, fragrances, or preservatives, that might be triggering the patient's symptoms. By identifying the specific irritant or allergen, appropriate management can be initiated, which can include advising the patient to avoid the identified substances.

Other options, such as VDRL serology, KOH prep, and Gram's stain, are not relevant diagnostic approaches for contact dermatitis. VDRL serology is used for syphilis screening, KOH preparation is for fungal infections, and Gram stain is utilized to detect bacterial infections. Thus, these tests do not provide the necessary information for diagnosing contact dermatitis, making patch testing the most suitable choice in this scenario.