**Core Concept**
Vesico-ureteric reflux (VUR) is a condition where urine flows retrogradely from the bladder to the ureters, potentially leading to renal damage and scarring. The investigation of choice is crucial in diagnosing VUR and assessing its severity.

**Why the Correct Answer is Right**
The investigation of choice for VUR is the **Voiding Cystourethrogram (VCUG)**. During VCUG, a contrast agent is instilled into the bladder through a catheter while the patient urinates. X-rays are taken to visualize the flow of urine and the reflux of contrast into the ureters. This allows for the assessment of the severity of VUR, which is graded from I to V according to the Gross Asymptomatic Vesicoureteral Reflux (GARF) grading system.

**Why Each Wrong Option is Incorrect**
**Option A:** Intravenous Pyelogram (IVP) is not the investigation of choice for VUR as it primarily evaluates the upper urinary tract and does not directly visualize the reflux of urine into the ureters.

**Option B:** Ultrasonography is useful for detecting hydronephrosis and assessing renal function but does not directly diagnose VUR.

**Option C:** Radionuclide Cystography (RNC) is an alternative to VCUG but is less commonly used due to its lower sensitivity and specificity for detecting VUR.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the diagnosis of VUR often requires a combination of imaging modalities, including ultrasound and VCUG, to accurately assess the severity of reflux and guide treatment.

**Correct Answer: C. Radionuclide Cystography (RNC).**