**Core Concept**
Vesico-ureteric reflux (VUR) is a condition where urine flows backward from the bladder into the ureters or kidneys, potentially leading to kidney damage. To diagnose VUR, a radiological procedure that visualizes the flow of urine during voiding is necessary.

**Why the Correct Answer is Right**
A micturition cystourethrogram (MCUG) is a fluoroscopic study that involves filling the bladder with a contrast agent and then imaging the patient while they urinate. This allows the radiologist to visualize the flow of urine and assess for any reflux into the ureters or kidneys. The MCUG is particularly useful for detecting VUR, as it can demonstrate the direction and severity of the reflux. During the procedure, the patient is typically asked to urinate while the fluoroscope captures images of the bladder and ureters, providing valuable information about the presence and extent of VUR.

**Why Each Wrong Option is Incorrect**
**Option A:** An ascending pyelogram involves injecting a contrast agent into the renal pelvis to visualize the upper urinary tract. While it can show obstructions or other issues in the upper urinary tract, it is not designed to assess vesico-ureteric reflux.

**Option B:** A cystogram involves filling the bladder with a contrast agent to visualize the bladder and urethra. While it can show bladder abnormalities or urethral strictures, it does not specifically assess for vesico-ureteric reflux.

**Option C:** An intravenous urogram involves injecting a contrast agent into a vein to visualize the entire urinary tract, from the kidneys to the bladder. While it can show obstructions or other issues in the urinary tract, it is not designed to specifically assess for vesico-ureteric reflux.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that a micturition cystourethrogram (MCUG) is the gold standard for diagnosing vesico-ureteric reflux, particularly in children. The MCUG can help identify the presence and severity of VUR, which is crucial for guiding treatment and preventing long-term kidney damage.

**✓ Correct Answer: D. Micturition cystourethrogram**