Photo Rounds

Rash on trunk and upper arms

A 16-year-old boy presented to his family physician (FP) with a 2-week history of a rash on his trunk and upper arms. The patient said the rash was slightly itchy, but only at night. He denied any history of recent respiratory infections or fever, and denied being sexually active. He continued attending school because he was otherwise feeling well.

What’s your diagnosis?


The FP recognized a “Christmas tree” pattern on the patient’s back, which led to a diagnosis of pityriasis rosea. Because there are no lab tests to confirm pityriasis rosea, it is usually a clinical diagnosis.

The Christmas tree pattern that signals pityriasis rosea is caused by the long axis of the plaques following the skin lines of the back. While it is a helpful indicator of pityriasis rosea when present, most cases do not show this pattern. In this case, a collarette scale pattern was also visible. This is seen when scaling is visibly raised on the edge of the plaques, like the collar of a shirt.

Pityriasis rosea can be difficult to distinguish from secondary syphilis, which can also present as a papulosquamous eruption. Therefore, taking a sexual history is important when a diagnosis of pityriasis rosea is being considered. In patients with a history of sexually transmitted diseases or high-risk sexual practices, a rapid plasma reagin test should be ordered.

The FP assured the patient and his mother that the pityriasis rosea rash was not contagious, would not cause scarring, and would resolve in one to 2 months without any treatment. The patient declined any medication for the pruritus. At a subsequent visit for a sports physical, the skin was found to be completely clear with no evidence of scarring.

Photos and text for Photo Rounds Friday courtesy of Richard P. Usatine, MD. This case was adapted from: Henderson D, Usatine R. Pityriasis rosea. In: Usatine R, Smith M, Mayeaux EJ, et al, eds. Color Atlas of Family Medicine. 2nd ed. New York, NY: McGraw-Hill; 2013: 896-900.

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