A 52-year-old man complains of pain in his back and fatigue for 6 months. He admits to polyuria and polydipsia. An X-ray film of the upper torso reveals numerous lytic lesions in the lumbar vertebral bodies. Laboratory studies show hypoalbuminemia and mild anemia and thrombocytopenia. A monoclonal immunoglobulin peak is demonstrated by serum electrophoresis, and a bone marrow aspiration demonstrates numerous atypical plasma cells. Urinalysis shows 4+ proteinuria. A renal biopsy in this patient would most likely show deposits of which of the following amyloid precursor proteins?

AL amyloid usually consists of the variable region of immunoglobulin light chains and can be derived from either the kappa (k) or lambda (l) moieties. Since the light chains produced by the neoplastic cells in plasma cell dyscrasias are unique to each patient, AL amyloid isolated from different persons differs in its amino acid sequence. AL protein is common to primary amyloidosis and amyloidosis associated with multiple myeloma, B-cell lymphomas, or other plasma cell dyscrasias. Multiple myeloma is accompanied by amyloidosis in 10% to 15% of cases. The other choices do not involve immunoglobulins.Diagnosis: Multiple myeloma