A 46-year-old Caucasian male presented with a classic case of chronic myelogenous leukemia. However, he failed to respond to imatinib (Gleevec) in both traditional and dose-escalating regimens. After failure of nilotinib (Tasigna), mutational analysis sequencing of the kinase domain using nested polymerase chain reaction was performed. This study revealed the resistance-related T315I mutation. At the same time, the patient suddenly entered an accelerated phase of the disease, and continuation of either monotherapy or combination therapy with readily available tyrosine kinase inhibitors was felt to be ill-advised. Thus, the patient was treated with an alternate modality with good initial hematological response. Approximately 2 ½ years later, while hematologically stable, the patient developed a solitary, exquisitely painful ulceration, 7 cm × 5 cm, located on the mid-medial foreleg. There was never any evidence of venous insufficiency. The patient denied the possibility of a spider bite.
The most likely cause of this lesion is: