**Core Concept**
The underlying condition in this scenario is likely a ureteropelvic junction obstruction (UPJO), characterized by an obstruction at the junction between the ureter and renal pelvis, leading to hydronephrosis and renal dysfunction.

**Why the Correct Answer is Right**
UPJO is typically managed surgically to relieve the obstruction and prevent further renal damage. In this case, the large hydronephrosis, cortical thinning, and significant impairment of kidney function (19% differential function) indicate that the obstruction is severe and requires intervention. The normal ureter suggests that the obstruction is indeed at the UPJ. Surgery, such as a pyeloplasty, would be the best course of action to restore renal function and alleviate symptoms.

**Why Each Wrong Option is Incorrect**
* **Option A:** Medical management with diuretics and pain relief would not address the underlying obstruction and may even worsen renal function.
* **Option B:** Observation and repeated ultrasonography may delay necessary surgical intervention, allowing further renal damage.
* **Option C:** Stenting of the ureter may provide temporary relief but does not address the underlying obstruction at the UPJ.

**Clinical Pearl / High-Yield Fact**
UPJO is a classic cause of intermittent flank pain and hydronephrosis in children, and prompt surgical intervention is crucial to prevent long-term renal damage.

**Correct Answer: C. Stenting of the ureter may provide temporary relief but does not address the underlying obstruction at the UPJ.