Image of the Week: What is the cause of this pustular rash?

An 80-year-old female with a history of arthritis and high blood pressure presents with recurrent lesions and erythema on the palms of her hands and the soles of her feet. On examination, hyperkeratosis, brown macules, and discrete pustules are seen on her palmoplantar surfaces. A fungal culture and serologic tests are unremarkable. ...