**Core Concept**
The infant's presentation of severe dehydration, diarrhea, flank mass, and hematuria suggests an obstructive uropathy, likely due to a congenital anomaly or an acquired condition causing urinary tract obstruction.

**Why the Correct Answer is Right**
The correct answer is likely **Pyelonephritis** secondary to a **Ureterocele**. A ureterocele is a congenital anomaly where the distal ureter balloons into the bladder, causing obstruction and potential pyelonephritis, especially in infants. The severe dehydration and hematuria can be due to the obstructive uropathy and subsequent pyelonephritis.

**Why Each Wrong Option is Incorrect**
**Option A:** **Nephrolithiasis** is unlikely in infants and doesn't explain the sudden onset of symptoms.
**Option B:** **Renal vein thrombosis** can cause hematuria, but it doesn't typically present with a flank mass or severe dehydration as primary symptoms.
**Option C:** **Pyelonephritis** without a ureterocele would not typically cause a palpable flank mass.

**Clinical Pearl / High-Yield Fact**
In infants with severe dehydration and hematuria, a palpable flank mass should raise suspicion for an obstructive uropathy, such as a ureterocele, and prompt further investigation.

**Correct Answer: C. Pyelonephritis**