A 66-year-old man has progressive fatigue and oedema. Haemoglobin is 8.4 g/dL, total protein 11.2 g/dL with albumin 3.0 g/dL, creatinine 5.6 mg/dL, and corrected calcium 11.8 mg/dL. Urine dipstick for albumin traces, yet the heat coagulation test and sulfosalicylic acid test show 4+ protein. Urine microscopy shows broad waxy casts. What is the most likely cause of his renal failure?

• A Focal segmental glomerulosclerosis
• B Minimal change disease with superimposed AKI
• C Amyloid AL deposits in glomeruli
• D Light chain cast nephropathy of multiple myeloma ✓

Explanation

The discordance between a trace dipstick (which detects albumin) and strongly positive sulfosalicylic acid testing (which detects any protein) points to massive monoclonal free light chain excretion. With anaemia, hypercalcaemia, raised globulin gap, and waxy casts, cast nephropathy of myeloma is the diagnosis: light chains bind Tamm-Horsfall protein and obstruct distal tubules. Minimal change disease and FSGS would give a strongly albumin-positive dipstick, and amyloidosis typically shows nephrotic-range dipstick-positive proteinuria rather than tubular casts.

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.