The correct answer is gouty tophus (choice “b”).
Gout is a defect of purine metabolism, usually caused by underexcretion of uric acid. Diet and heredity also play parts in gout’s development. Gouty tophi usually develop after years of hyperuricemia. As uric acid builds up in the bloodstream over time, it can then begin to be deposited into joints—most commonly the first metatarsal-phalangeal—as well as cartilage or even bones.
On further questioning, the patient recalled having been told on several occasions that his serum uric acid was elevated. In retrospect, his arthritis was most likely gouty in nature.
In terms of the differential, BCC (choice “a”) is common on helical rims, but it would not have contained the type of material found in this patient’s lesion. Also, it would not have waxed and waned as this lesion had done.
Epidermal cysts (choice “c”) can certainly come and go in prominence, but they are filled with a cheesy, pasty material—not the dry crystalline substance found in this lesion. Moreover, most epidermal cysts will have a small comedonal punctum over the center of the lesion. Dystrophic calcification (choice “d”) can mimic gouty tophi, but it is usually rough, firm, and fixed. It certainly would not be coming and going as it pleases.
Surgical excision of the tophus was offered, but the patient was content with knowing the correct diagnosis. His PCP had previously explained therapeutic options—such as medication and dietary changes—that could address the overall problem. The patient elected to pursue treatment with his PCP.