**Core Concept**
Obstructive jaundice is a condition where bilirubin accumulation occurs due to a blockage in the bile ducts. Ultrasonography (USG) is a valuable diagnostic tool used to identify the cause of obstructive jaundice, particularly in cases where the obstruction is due to a mechanical blockage.

**Why the Correct Answer is Right**
In a patient with obstructive jaundice, USG can accurately diagnose the cause by visualizing the bile ducts and identifying any mechanical obstructions such as gallstones, tumors, or bile duct strictures. USG can also assess the liver and bile ducts for signs of inflammation, scarring, or dilation. Additionally, USG can guide the placement of percutaneous transhepatic cholangiography (PTC) or endoscopic retrograde cholangiopancreatography (ERCP) to confirm the diagnosis and provide a therapeutic option.

**Why Each Wrong Option is Incorrect**
* **Option A:** USG can be used to assess the liver and bile ducts for signs of inflammation, scarring, or dilation, making it a useful tool for diagnosing obstructive jaundice.
* **Option B:** USG can accurately diagnose the cause of obstructive jaundice by visualizing the bile ducts and identifying any mechanical obstructions.
* **Option D:** USG can guide the placement of PTC or ERCP to confirm the diagnosis and provide a therapeutic option.

**Clinical Pearl / High-Yield Fact**
When using USG to diagnose obstructive jaundice, it is essential to look for signs of bile duct dilation, such as a dilated common bile duct or a dilated intrahepatic bile ducts, which can indicate a mechanical obstruction.

**Correct Answer:** C