**Core Concept**
The presence of blood in the urine, specifically in the form of red blood cells (RBCs), casts, and deformed RBCs, suggests a glomerular origin of the hematuria. This is because glomerular damage leads to the presence of RBCs and casts in the urine, which are formed in the renal tubules.

**Why the Correct Answer is Right**
The presence of RBC casts, which are formed in the renal tubules, and deformed RBCs, indicates a glomerular source of bleeding. This is consistent with glomerulonephritis, an inflammatory process affecting the glomeruli. The glomeruli are the filtration units of the kidneys, and their damage leads to the presence of blood and other cellular elements in the urine.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because pyelonephritis, an infection of the kidney, would more likely present with symptoms of infection such as fever, flank pain, and dysuria, rather than isolated hematuria.

**Option B:** This option is incorrect because nephrolithiasis, or kidney stones, would more likely present with severe flank pain, hematuria, and possibly obstructive symptoms such as urinary frequency and urgency.

**Option C:** This option is incorrect because polycystic kidney disease (PKD) is a genetic disorder that affects the kidneys, but it typically presents with a long history of hematuria, hypertension, and renal failure, rather than isolated hematuria.

**Clinical Pearl / High-Yield Fact**
RBC casts are a specific indicator of glomerular disease and are often seen in conditions such as glomerulonephritis, lupus nephritis, and Goodpasture's syndrome. The presence of RBC casts in the urine should prompt further investigation into the underlying cause of the hematuria.

**Correct Answer:** C. Glomerulonephritis.