A 69-year-old woman with a known history of multiple myeloma presents to the emergency department for worsening fatigue and a 5-lb weight loss. She has been taking over-the-counter ibuprofen for chronic rib pain. Past medical history is otherwise negative. Vital signs are within normal limits. Physical examination shows pale conjunctivae and tenderness over the mid-back.
Laboratory studies show the following:
Hemoglobin: 9.0 g/dL
Calcium: 11.2 mg/dL
Creatinine: 3.4 mg/dL (Baseline: 1.1 mg/dL)
Urinalysis: 1+ protein on dipstick; microscopic examination shows no cells or casts.
24-hour urine protein: 4.2 g/day (predominantly non-albumin protein)
Which of the following is the most likely mechanism of this patient’s acute kidney injury?
The correct answer is:
A) Formation of intratubular obstructive casts
Myeloma kidney, or cast nephropathy, is the most common cause of kidney injury in multiple myeloma. The underlying pathology involves the overproduction of monoclonal free light chains (Bence-Jones proteins). These light chains are filtered by the glomerulus and reach the distal convoluted tubules. In the distal tubule, the light chains bind to Tamm-Horsfall protein (uromodulin) under acidic conditions. This interaction forms large, waxy, eosinophilic intratubular casts that obstruct the tubular lumen and exert direct cytotoxic effects on the tubular epithelium, leading to inflammation and giant cell reactions. A classic clue on the USMLE is the mismatch between the urine dipstick and the 24-hour urine protein collection. A urine dipstick primarily detects albumin. In cast nephropathy, the dipstick is often negative or only trace-positive because the protein being lost is light chains, not albumin. A sulfosalicylic acid (SSA) test detects all proteins and will be strongly positive in patients with cast nephropathy from multiple myeloma.
Answer choice B: Glomerular deposition of insoluble amyloid fibrils, is incorrect. While AL amyloidosis can occur in myeloma, it typically presents with nephrotic syndrome (heavy albuminuria, edema) due to glomerular involvement. This patient has tubular obstruction, not glomerular basement membrane deposition.
Answer choice C: Immune-complex mediated proliferative glomerulonephritis, is incorrect. This mechanism is seen in conditions like lupus or post-streptococcal glomerulonephritis. Myeloma-related renal injury is typically non-inflammatory at the glomerular level.
Answer choice D: Nephrotoxicity due to nonsteroidal anti-inflammatory drugs (NSAIDs), is incorrect. While this patient is taking ibuprofen, which can cause acute interstitial nephritis or papillary necrosis, the massive discrepancy between her dipstick (1+) and her total protein (4.2g) is pathognomonic for light-chain cast nephropathy.
Answer choice E: Pre-renal azotemia due to hypercalcemia-induced diuresis, is incorrect. Hypercalcemia causes a nephrogenic diabetes insipidus-like effect leading to dehydration. While this may exacerbate the kidney injury, it does not explain the high levels of non-albumin protein in the urine.
Key Learning Point
Myeloma kidney (cast nephropathy) is caused by the precipitation of Bence-Jones proteins with Tamm-Horsfall protein in the distal tubules. It characteristically presents with acute renal failure and a negative/trace urine dipstick despite high levels of total protein on a 24-hour collection or SSA test.
