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:

Multiple myeloma Multiple myeloma is the most likely etiology. The combination of hypercalcemia and acute renal failure raises the possibility of multiple myeloma as the bone breakdown secondary to tumor involvement releases large amounts of calcium to the extracellular fluid and hypercalcemia ensues. The renal failure in myeloma is primarily related to hypercalcemia combined with proteinaceous cast formation within the renal tubules, producing a form of intratubu- lar obstruction as well as a tubular inflammatory lesion. The major diagnostic clue is the finding of dipstick mildly positive urine with a sulfosalicylic acid strongly positive urine. Dipstick testing does not detect the negatively charged light-chain proteins, only the alburnin. Renovascular lesions and thrombotic renal disease could present with this picture, although they should not be asso- ciated with hypercalcemia and severe back pain, and findings upon examination of the urine would not include proteins. Systemic lupus erythematosus (SLE) can, of course, be associated with severe anemia and joint mani- festations, but hypercalcemia is not pa of the picture.