A 65-year-old man with newly diagnosed multiple myeloma has serum calcium 13.2 mg/dL, haemoglobin 9.1 g/dL, and creatinine 2.4 mg/dL. Urine dipstick is negative for protein despite 4+ protein on sulfosalicylic acid test. Which mechanism BEST explains the renal impairment?

• A Monoclonal immunoglobulin deposition disease
• B Hypercalcaemia-induced pre-renal azotaemia
• C AL amyloidosis
• D Light chain cast nephropathy (myeloma kidney) ✓

Explanation

Light chain cast nephropathy is the most common cause of renal failure in myeloma. Filtered free light chains precipitate with Tamm-Horsfall protein in distal tubules, forming obstructive casts with surrounding giant cell reaction. The negative dipstick for protein (which detects albumin) with positive sulfosalicylic acid (which detects all protein including light chains) is classic. Hypercalcaemia contributes but cast nephropathy is the dominant mechanism of AKI in this setting.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.