Question

What is the likely source of hematuria that has been persisting for the past 3 days, with red cell casts evident on urinalysis?

• A. Bladder
• B. Urethra
• C. Kidney
• D. Ureter

Hint:

Only kidney tubules can mould red blood cells into a cast.

Answer 1

The Correct Option is C

This question checks whether we know what a red cell cast tells us about the site of bleeding in the urinary tract.

1. Bladder: bladder bleeding gives loose red cells or clots in the urine, never a moulded cast, because the bladder has no tubules.
2. Urethra: urethral bleeding shows as blood staining at the start of the stream, also with no cast, since the urethra is just a straight passage.
3. Kidney: only the kidney has the narrow tubules that can shape red blood cells into a solid cylindrical cast before they wash out into the urine. Seeing a red cell cast means bleeding started here.
4. Ureter: like the bladder and urethra, the ureter is a simple tube with no tubular mould, so it cannot produce a cast either.

Since casts can form only inside the kidney's nephrons, red cell casts on urinalysis point straight to a renal source of the hematuria, often glomerular disease.

Let's summarize:

• Casts are made only inside renal tubules.
• The bladder, ureter, and urethra cannot produce true casts.

So the correct answer is the kidney.