## **Core Concept**
RBC (Red Blood Cell) casts are a type of urinary cast that indicates the presence of bleeding within the kidneys themselves, specifically from the glomeruli or tubules. They are a key diagnostic feature in certain renal pathologies.

## **Why the Correct Answer is Right**
The presence of RBC casts in urine is a hallmark of glomerulonephritis, which is an inflammation of the glomeruli—the filtering units of the kidneys. This condition can lead to hematuria (blood in the urine) and proteinuria (excess proteins in the urine). The correct answer, Glomerulonephritis, is associated with RBC casts because the bleeding originates from within the glomeruli, and the casts form in the renal tubules.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because while it might be related to kidney issues, it's not specified and does not directly relate to the presence of RBC casts as a diagnostic criterion.
- **Option B:** This option does not directly correlate with the presence of RBC casts. Conditions not primarily involving glomerular injury are less likely to produce RBC casts.
- **Option C:** Similarly, this option does not directly relate to the conditions characterized by the presence of RBC casts.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that RBC casts are almost exclusively indicative of a glomerular or tubulointerstitial origin of hematuria, distinguishing it from lower urinary tract bleeding. This helps in localizing the source of bleeding within the urinary tract.

## **Correct Answer Line**
**Correct Answer: C. Glomerulonephritis.**