A 65-year-old woman comes to the physician for a one-year history of generalized fatigue. She has no significant past medical history. Laboratory studies are as follows:
Hemoglobin 14 g/dL
MCV 108 fL
Serum Ca2+ 9.3 mg/dL
Serum kappa light chains 320 mg/dL (NR 3.3-19.4)
ESR Normal
Bone marrow biopsy shows 11% plasma cells. This patient is at greatest risk for the development of which of the following?