An adult patient presented with a slow-growing "knot" on the volar aspect of the left thumb. The patient first noticed the nodule approximately one year earlier. On the day before presentation, a small amount of white drainage oozed from the nodule after minimal trauma to the overlying skin. There was minimal pain and no numbness, tingling, erythema, or warmth associated with the nodule. The patient had no previous related injury or foreign body in the affected area.
Physical examination revealed a small, firm nodule on the volar aspect of the interphalangeal joint of the left thumb. The overlying skin was broken, revealing pearl-gray material (Figures 1 and 2). Mild tenderness to palpation was present only over the area of disrupted skin. The patient had full extension of the interphalangeal joint, but flexion was slightly restricted because of the mass. The patient had full strength with flexion of the interphalangeal and metacarpophalangeal joints. Hand radiographs were significant for soft tissue swelling at the joint. No bony infiltration was apparent.
Based on the patient's history and physical examination, which one of the following is the most likely diagnosis?
 A. Epidermal inclusion cyst.
 B. Ganglion cyst.
 C. Giant cell tumor of the tendon sheath.
 D. Schwannoma.
 E. Verruca vulgaris.
The answer is A:...