A 62-year-old man presents with fatigue and worsening creatinine over 6 weeks (baseline 1.0 to 3.8 mg/dL). Examination shows pallor and bony tenderness of the spine. Hemoglobin is 7.9 g/dL, corrected calcium 12.4 mg/dL, and total protein 10.2 g/dL with albumin 3.0 g/dL. Urinalysis shows trace protein with no cellular casts, but the dipstick underestimates proteinuria. Which finding on urine microscopy best explains his kidney injury?

• A Bence Jones protein precipitating with Tamm-Horsfall protein to form obstructing casts ✓
• B Oxalate crystals within tubular lumina causing intratubular obstruction
• C Granular casts consistent with ischemic acute tubular necrosis
• D Waxy casts indicating chronic glomerular scarring

Explanation

The picture of anemia, hypercalcemia, hyperproteinemia with low albumin, and rising creatinine points to multiple myeloma with light chain cast nephropathy. Free monoclonal light chains bind Tamm-Horsfall protein in distal tubules, forming large brittle casts that obstruct and injure tubular epithelium. The dipstick detects only albumin, so it shows only trace protein despite heavy light chainuria. Oxalate crystals suggest ethylene glycol injury, which does not fit this presentation.

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.