A female with pruritic, vesicular lesions on her forearms likely has which type of dermatitis?

The presence of pruritic, vesicular lesions on the forearms is indicative of contact dermatitis, specifically allergic contact dermatitis. Patch testing is the appropriate choice for diagnosing this condition, as it is specifically designed to identify allergens that may be causing a localized allergic reaction. In patch testing, small amounts of various allergens are applied to the skin, allowing practitioners to observe if any of them provoke a dermatitis-like reaction, which aligns well with the symptoms described.

In contrast, the other options are various laboratory and diagnostic tests used for different purposes. VDRL serology is primarily used for screening syphilis and does not pertain to skin symptoms. KOH prep is a diagnostic test for fungal infections, allowing assessment of skin scrapings under a microscope to identify fungal elements, which does not directly correlate to allergic contact dermatitis. Gram's stain is utilized to identify bacterial infections through staining and microscopic examination of bacterial cultures. Thus, none of these options would give insight into lesions caused by allergic reactions, making patch testing the most relevant diagnostic tool in this scenario.