**Core Concept:** Vesico-ureteral reflux (VUR) is a condition characterized by the backward flow of urine from the bladder into the ureter, leading to potential renal damage. This can be investigated using imaging techniques such as voiding cystourethrogram (VCUG) or ultrasonography.

**Why the Correct Answer is Right:**

The correct answer is **D. Voiding Cystourethrogram (VCUG)**. A VCUG is the gold standard investigation for demonstrating VUR, as it visualizes the ureter during the filling and voiding phases, allowing for accurate assessment of reflux. VCUG involves injecting a radiopaque contrast into the bladder, which is then visualized on X-ray to observe the reflux patterns.

**Why Each Wrong Option is Incorrect:**

A. **Option A. Urinary tract ultrasound (UTUS):** UTUS is useful for assessing renal and ureteral abnormalities but is not specific for VUR detection, as it lacks the dynamic assessment provided by VCUG.

B. **Option B. Urethrography:** Urethrography is a radiographic procedure that visualizes the urethra and penile urethra but is not suitable for detecting VUR, as it is static and does not demonstrate the ureteral anatomy during the filling and voiding phases.

C. **Option C. Urine culture and sensitivity:** Although urine culture and sensitivity are essential for evaluating urinary tract infections, they do not directly demonstrate VUR.

**Clinical Pearl:**

Voiding cystourethrogram (VCUG) is the preferred method for detecting and grading VUR, as it allows for dynamic visualization of the ureteral anatomy and reflux patterns. This information aids in early diagnosis, treatment planning, and monitoring of VUR patients.