Image of the Week: What's causing this elderly woman's weight loss?

A 65-year-old woman presents to a clinician for the first time in 25 years with a history of weight loss and weakness. On examination, she is hypertensive and her extremities demonstrate pitting edema. Urinalysis shows proteinuria. An abdominal fat pad tissue biopsy reveals deposits of amyloid, and further immunochemical staining demonstrates serum amyloid A (SAA) proteins. Which of the following ...