**Core Concept**
Cholestasis of pregnancy is a condition characterized by impaired bile flow, leading to pruritus, jaundice, and elevated liver enzymes. It is a significant cause of fetal distress and preterm labor. The diagnosis of cholestasis of pregnancy relies on a combination of clinical, laboratory, and imaging tests.

**Why the Correct Answer is Right**
The best diagnostic test for cholestasis of pregnancy is serum bile acid levels. Elevated bile acid levels are a sensitive and specific marker for cholestasis of pregnancy. This is because bile acids are not conjugated in the liver, and their elevation indicates impaired bile flow. Serum bile acid levels can be used to monitor disease activity and guide treatment.

**Why Each Wrong Option is Incorrect**
**Option A:** Liver function tests (LFTs) such as alkaline phosphatase and gamma-glutamyl transferase (GGT) may be elevated in cholestasis of pregnancy, but they are not specific and can be abnormal in other conditions. LFTs are not the best diagnostic test for cholestasis of pregnancy.
**Option B:** Ultrasound may show gallstones or bile duct dilation, but it is not a specific test for cholestasis of pregnancy. Ultrasound findings can be normal in many cases.
**Option C:** Pruritus is a common symptom of cholestasis of pregnancy, but it is not specific and can be present in other conditions. Pruritus is a clinical finding, not a diagnostic test.

**Clinical Pearl / High-Yield Fact**
In patients with suspected cholestasis of pregnancy, it is essential to monitor serum bile acid levels regularly, as they can fluctuate over time. A bile acid level above 10 micromoles per liter is often used to diagnose cholestasis of pregnancy.

**Correct Answer:** B. Ultrasound may show gallstones or bile duct dilation, but it is not a specific test for cholestasis of pregnancy. Ultrasound findings can be normal in many cases.