Urinalysis shows RBC casts; likely source is :
**Core Concept**
RBC casts are a type of urinary cast that indicates the presence of bleeding into the nephrons, specifically the glomeruli, of the kidneys. They are a hallmark of glomerulonephritis, an inflammatory disorder of the glomeruli that can be caused by various factors, including immune-mediated diseases, infections, and vasculitis.
**Why the Correct Answer is Right**
RBC casts form when red blood cells are trapped in the tubules of the nephron and then cast out into the urine. This occurs when the glomerular filtration barrier is compromised, allowing red blood cells to leak into the nephron. The presence of RBC casts suggests that the bleeding is originating from within the nephron, specifically from the glomeruli. This is in contrast to hematuria caused by bleeding from the ureters, bladder, or urethra, which would not produce RBC casts.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because RBC casts are not typically associated with conditions that cause bleeding from the lower urinary tract, such as urethritis or cystitis.
**Option B:** This option is incorrect because while pyelonephritis can cause hematuria, it is not typically associated with RBC casts, which are more specific for glomerular bleeding.
**Option C:** This option is incorrect because while renal infarction can cause hematuria, it is not typically associated with RBC casts, which are more specific for glomerular bleeding.
**Clinical Pearl / High-Yield Fact**
The presence of RBC casts in the urine is a key diagnostic feature of glomerulonephritis, and it can be a useful differentiator from other causes of hematuria.
**Correct Answer:** C.