A 62-year-old man presents with fatigue, bone pain, and serum creatinine 4.8 mg/dL. Hb 8.5 g/dL, calcium 12.8 mg/dL, total protein 9.2 g/dL, albumin 2.8 g/dL. Serum protein electrophoresis shows an M-spike. Urine dipstick is trace positive for protein but 24-hour urine shows 3.5 g proteinuria. Which mechanism best explains the kidney injury?
- A Cast nephropathy from filtered light chains damaging tubules ✓
- B AL amyloidosis with glomerular amyloid deposition
- C Monoclonal immunoglobulin deposition disease
- D Hypercalcemia-induced pre-renal acute kidney injury
Correct answer: A. Cast nephropathy from filtered light chains damaging tubules
Explanation
Cast nephropathy (myeloma kidney) is the most common cause of kidney injury in multiple myeloma. Filtered monoclonal light chains precipitate in distal tubules forming obstructive casts with giant cell reaction. Dipstick detects albumin, not light chains, so the discordance between dipstick and 24-hour urine protein is characteristic. This is the most common mechanism of myeloma-related AKI.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.