**Core Concept**
The patient's symptoms, including abdominal pain, jaundice, elevated alkaline phosphatase, and conjugated hyperbilirubinemia, along with a blocked biliary tree on ultrasound, suggest an obstruction in the biliary system. This obstruction can be caused by various factors, including gallstones, tumors, or inflammation.

**Why the Correct Answer is Right**
The most likely cause of this patient's symptoms is a gallstone obstructing the common bile duct. This obstruction leads to a buildup of conjugated bilirubin, resulting in jaundice and elevated alkaline phosphatase levels. The biliary tree blockage on ultrasound supports this diagnosis.

**Option A:**
Option A, **primary biliary cirrhosis**, is a chronic liver disease characterized by progressive destruction of the bile ducts within the liver. While it can cause elevated alkaline phosphatase and conjugated hyperbilirubinemia, it is not typically associated with a sudden onset of symptoms or a blocked biliary tree on ultrasound.

**Option B:**
Option B, **cholelithiasis**, refers to the presence of gallstones within the gallbladder. While gallstones can cause biliary obstruction, the symptoms described suggest an obstruction at the level of the common bile duct, which is more consistent with a gallstone migrating from the gallbladder into the bile duct.

**Option C:**
Option C, **carcinoma of the pancreas**, can cause biliary obstruction by invading the common bile duct. However, the patient's symptoms and ultrasound findings do not provide sufficient evidence to support this diagnosis as the most likely cause.

**Option D:**
Option D, **choledocholithiasis**, refers to the presence of a gallstone within the common bile duct. This condition can cause the symptoms described, including jaundice, elevated alkaline phosphatase, and conjugated hyperbilirubinemia, along with a blocked biliary tree on ultrasound.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **conjugated hyperbilirubinemia** is often associated with post-hepatic causes of jaundice, such as biliary obstruction. In contrast, **unconjugated hyperbilirubinemia** is more commonly associated with pre-hepatic causes, such as hemolysis or increased bilirubin production.

**Correct Answer:** D. Cholelithiasis within the common bile duct.