A 64-year-old man developed persistent back pain. On examination, his BP is 120/80, pulse 70, respiratory rate is 15, and temp is 98.6degF. Physical findings include severe pallor & evidence of muscle wasting. On urinalysis, 4 + protein. M/E of urine reveals occasional broad and occasional granular cast. Lab studies: Serum creatinine = 5.1 mg/dl, serum Na = 141, K = 5.6, chloride = 101, CO2 = 14, serum calcium = 11.7, and serum phosphorus = 6.0. Most likely etiology is: