**Core Concept**
Obstructive jaundice is a clinical condition characterized by the obstruction of bile flow, leading to an accumulation of bilirubin in the blood. This condition can be caused by various factors, including gallstones, tumors, and strictures in the bile ducts. The investigation of choice for diagnosing the underlying cause of obstructive jaundice is crucial for planning effective treatment.

**Why the Correct Answer is Right**
The investigation of choice for obstructive jaundice is Endoscopic Retrograde Cholangiopancreatography (ERCP). ERCP is a minimally invasive procedure that combines endoscopy and fluoroscopy to visualize the bile and pancreatic ducts. It allows for the direct visualization of the obstruction, sampling of bile for cytology and culture, and placement of stents to relieve the obstruction. ERCP is particularly useful in cases where percutaneous transhepatic cholangiography (PTC) or magnetic resonance cholangiopancreatography (MRCP) are contraindicated or unsuccessful.

**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasound is a useful initial investigation for obstructive jaundice, but it is not the investigation of choice for diagnosing the underlying cause of the obstruction.
**Option B:** CT scan is a useful investigation for detecting the cause of obstructive jaundice, but it is not as sensitive as ERCP for visualizing the bile and pancreatic ducts.
**Option C:** PTC is a useful investigation for diagnosing the cause of obstructive jaundice, but it is more invasive than ERCP and carries a higher risk of complications.

**Clinical Pearl / High-Yield Fact**
ERCP is a valuable investigation for diagnosing and treating obstructive jaundice, but it carries a higher risk of complications, including pancreatitis and bleeding. It is essential to carefully select patients for ERCP and to have a high level of expertise in performing the procedure.

**Correct Answer: C. Endoscopic Retrograde Cholangiopancreatography (ERCP).**