**Core Concept**
Vesicoureteric reflux (VUR) is a condition where urine flows retrogradely from the bladder into the ureters or kidneys, potentially leading to urinary tract infections and kidney damage. The diagnosis of VUR relies on demonstrating the abnormal flow of urine from the bladder to the ureters or kidneys.

**Why the Correct Answer is Right**
The correct answer, **Voiding Cystourethrogram (VCUG)**, is a radiological investigation that uses fluoroscopy and contrast material to visualize the flow of urine from the bladder during urination. VCUG is considered the gold standard for diagnosing VUR, as it directly demonstrates the retrograde flow of urine from the bladder into the ureters or kidneys. The contrast material allows the radiologist to visualize the ureters and kidneys in real-time, confirming the diagnosis of VUR.

**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasonography is not sufficient to diagnose VUR, as it cannot directly visualize the flow of urine from the bladder to the ureters or kidneys.
**Option B:** Intraday renal scan, also known as DMSA scan, is used to detect pyelonephritis or renal scarring, but it is not a direct diagnostic tool for VUR.
**Option C:** CT urogram or CT excretory urogram is not typically used to diagnose VUR, as it is more sensitive for detecting stone disease or malignancies.

**Clinical Pearl / High-Yield Fact**
Mnemonic: "VCUG stands for 'Visualizing the Urine flow, Catching the reflux'".

**Correct Answer:** C. Voiding Cystourethrogram (VCUG).