A pruritic geometric patch typical of allergic contact dermatitis, and in this patient, the clonidine patch is the likely cause. Contact dermatitis is an example of an “outside job” when a rash is in linear or geometric patterns because the rash corresponds exactly to where the allergen touches the skin from the outside. Allergic contact dermatitis is a hypersensitivity reaction to a specific chemical. With repeated exposure to the chemical, a pruritic eczematous dermatitis develops on the area that was exposed. In exuberant cases, pinpoint flesh-colored to red papules develop in the vicinity of exposure or diffusely over the body. The clonidine patch has been associated with allergic contact dermatitis. Although many transdermal drug delivery systems may cause allergic contact dermatitis, the clonidine patch tends to have a higher incidence, possibly owing to its formulation that allows the patch to be left in place for up to 7 days. The patient's adjacent areas of hyperpigmentation likely represent resolving inflammation from prior applications of the patch.

A fixed drug eruption is typically a purple patch that is painful and is not localized to the area underneath a patch. Fixed drug eruptions occur in the same location (fixed) each time a patient is exposed to the same medication. Lips, genitals, and hands are commonly involved. Common drug culprits include over-the-counter medications such as pseudoephedrine, NSAIDs, sulfonamide medications, and other antibiotics.

Irritant contact dermatitis is a direct toxic effect on the epidermis from exposure to a chemical such as a cleaning agent, other caustic substance, or repeated wetting and drying and is not mediated by the immune system. Irritant contact dermatitis would present with a generalized dermatitis in all of the exposed areas but not in a geometric pattern.

Tinea corporis is a dermatophyte infection that may occur in healthy people, although patients with compromised immune systems (such as with diabetes mellitus) may be more susceptible. The classic findings are a pruritic, circular or oval, erythematous, scaling patch or plaque that spreads centrifugally with central clearing. This patient's skin findings are not consistent with this diagnosis.