During the month of December, a middle-aged patient with chronically dry skin develops widespread coin-shaped lesions, which begin as itchy patches of vesicles and papules, later ooze serum and crust over. These are most numerous on the extensor surfaces of the extremities and on the buttocks. He says that some of the lesions have appeared to heal and then reappear at the same sites. Most likely diagnosis?

The patient most likely has nummular dermatitis, which is a chronic inflammation of the skin, the etiology of which is still unknown. The presentation illustrated in the question stem is typical. This condition should be in your differential diagnosis whenever the terms "coin-shaped" or "discoid" are used to describe a patient's skin lesions in a question. Microscopically, the dominant feature is a localized spongiosis (corresponding to edema) of the epidermis, which may also contain minute fluid-filled holes that correspond to the tiny vesicles seen clinically in early lesions. Treatment of these patients is problematic, and numerous regimens involving coicosteroids or antibiotics have been recommended, each of which appears to work with some but not all patients. Pompholyx produces deep-seated pruritic vesicles on the palms, fingers, and soles. Psoriasis can produce coin-shaped lesions, but they are covered with silvery scale. Seborrheic dermatitis produces hyperkeratosis on the scalp and face and covered with greasy scales. Stasis dermatitis can produce discoloration and ulceration of the lower legs near the ankles. causes: Many contact sensitizers or irritants are known to cause contact dermatitis superimposed on nummular dermatitis. Studies have implicated nickel, cobalt, chromate, and fragrance as likely culprits. Xerosis, or dehydration of skin is also a likely cause Infection with Staphylococcus aureus bacteria or Candida albicans may also play a role Treatment: One of the keys to treatment and prevention involves keeping the skin moisturized. Lotions, creams, and bath oils may help prevent an outbreak. If the condition flares up, a common treatment involves the application of topical coicosteroids. Oral antihistamines may help lessen itching. Avoidance of irritants is a common strategy. More severe cases sometimes respond to ultraviolet light treatment. If the condition occurs only during the sun-less winter months then vitamin D supplement might be an effective treatment. Ref Harrison 20th edition pg 1234