**Core Concept**
Hydronephrosis is a condition characterized by the swelling of a kidney due to the accumulation of urine, often resulting from an obstruction in the urinary tract. The investigation of choice for hydronephrosis aims to diagnose the underlying cause and assess the severity of the condition.

**Why the Correct Answer is Right**
The investigation of choice for hydronephrosis is ultrasound (US), which provides a non-invasive and cost-effective assessment of renal morphology and function. US can detect hydronephrosis by visualizing the kidney's size, echogenicity, and the presence of any dilatation of the renal pelvis and calyces. Additionally, US can help identify potential causes of hydronephrosis, such as kidney stones, tumors, or ureteropelvic junction obstruction.

**Why Each Wrong Option is Incorrect**
**Option A:** CT scan is not the first-line investigation for hydronephrosis, although it may be used in some cases to provide additional information or to assess the severity of the condition.
**Option B:** MRI is not typically used as the primary investigation for hydronephrosis, although it may be useful in certain situations, such as assessing the presence of a tumor or evaluating the renal function in patients with complex urinary tract anomalies.
**Option C:** IVP (Intravenous Pyelography) is an older radiographic technique that involves injecting a contrast agent into the bloodstream to visualize the urinary tract. However, it is less sensitive and less specific than ultrasound for detecting hydronephrosis and is not typically used as the first-line investigation.

**Clinical Pearl / High-Yield Fact**
Ultrasound is the investigation of choice for hydronephrosis due to its non-invasive nature, lack of radiation exposure, and ability to provide real-time imaging of the kidneys and urinary tract.

**Correct Answer:** D. Ultrasound