Hematuria with red blood cell casts and mild proteinuria most strongly suggests disease in which renal structure?

• A. Glomerulus
• B. Renal pelvis
• C. Interstitium
• D. Renal tubules

Answer: (A)

Red blood cell casts in the urine indicate that red blood cells are entering the filtrate at the level of the kidney and then being carried down the tubules, where they form casts. This pattern points to bleeding from the glomerular filtration barrier, i.e., the glomerulus. The accompanying mild proteinuria fits with glomerular damage that increases permeability enough to let some protein through but not to the extent seen in nephrotic-range protein loss.

If the problem were in the renal pelvis or collecting system, you’d expect visible blood in the urine without RBC casts. Interstitial disease typically yields white blood cell casts or eosinophils, and tubular (intrinsic tubule) disease tends to produce granular or epithelial cell casts with tubular proteinuria rather than RBC casts. Thus the glomerulus is the structure most consistent with this presentation.