What is responsible for these asymptomatic, flat bumps?

A 55-year-old man presents with a 4-week history of asymptomatic flat bumps on the head of his penis only. They have slowly appeared, about a week apart, and have since gradually enlarged. He denies any other rash. He denies any new sexual exposure and is reportedly happily married.  

What is responsible for these asymptomatic, flat bumps?

A. Lichen planus
B. Human papillomavirus (HPV)
C. Psoriasis
D. Pityriasis rosea
E. Dermatophyte infection

(Answer on next page)

Answer: Lichen planus 

This patient has the flat, well-circumscribed, purplish papules typically seen with lichen planus. He did not show any oral lesions. He responded well to a midpotency steroid cream. 

Differential diagnosis. HPV is reasonable and can be confirmed by shave biopsy if uncertain of the diagnosis. Psoriasis is possible, though more scaling is usually seen. Pityriasis rosea can affect the penis but would affect other areas as well, and again, more scaling would be expected. Dermatophyte infection of the penis is rare, typically seen in immunocompromised hosts (eg, individuals with diabetes) and again, scaling would be expected.