In multiple myeloma, which urinary finding is a hallmark?

• A. Bence Jones proteinuria
• B. Casts of eosinophils
• C. Albuminuria
• D. Hematuria

Answer: (A)

Free light chains produced by malignant plasma cells are filtered by the kidneys and appear in urine as Bence Jones proteinuria. These monoclonal immunoglobulin light chains (κ or λ) are small enough to pass through the glomerulus and, when produced in excess, are not fully reabsorbed in the proximal tubules. They can cause renal injury by forming casts with Tamm-Horsfall protein in the tubules, a process known as myeloma kidney. This pattern of proteinuria is classically associated with multiple myeloma and is more specific than other types of urinary protein like albumin. Dipstick protein tests, which mainly detect albumin, may miss these light chains, so targeted tests such as urine protein electrophoresis or immunofixation are used to confirm. Other urinary findings listed, like eosinophil casts, point to different renal diseases; albuminuria and hematuria can occur but are not hallmark features of myeloma.