A 22-year-old man presented with a confluent, blistering rash on his torso. He had participated in a river float 2 days earlier, passing slowly through dappled shade for about 4 hours. Early in the river float, he spilled lime juice on his chest and abdomen; he was also shirtless and did not apply sunscreen. The day after the river float, he noticed mild discomfort and redness in an unusual distribution over his chest and abdomen. By that evening, the pain and redness had worsened, and the rash was raised. After he presented for evaluation, the blisters burst, releasing serous fluid. After 19 days, the pain had resolved completely, but erythema persisted (Figure 1).

QUESTION

Based on the patient's history and physical examination, which one of the following is the most likely diagnosis?

• A. Contact dermatitis.

• B. Phytophotodermatitis.

• C. Psoriasis.

• D. Sunburn.

• E. Tinea versicolor.

DISCUSSION

The answer is B: phytophotodermatitis, which is caused by long-wave ultraviolet (UV) radiation after sensitization by furocoumarins.¹ These substances are produced by a number of plant families and are found in the highest concentrations in celery, parsley, limes, and grapefruits. Furocoumarins are absorbed topically and intercalate with DNA, where they catalyze cross-linking in the presence of UV light, causing cell damage and cell death. In this case, the rash was consistent with the area of lime juice exposure, which was crucial in determining the cause of the rash.