**Core Concept**
Cholestasis of pregnancy is a condition characterized by impaired bile flow and elevated bile acids in the blood. It can lead to severe pruritus, fetal distress, and preterm labor. Accurate diagnosis and monitoring are crucial to manage the condition effectively.

**Why the Correct Answer is Right**
Serum bile acids levels are the investigation of choice in cholestasis of pregnancy because they directly reflect the degree of bile flow impairment. Elevated bile acids levels are a sensitive indicator of cholestasis, and their measurement helps in monitoring the response to treatment. Bile acids, such as taurocholic acid and glycocholic acid, are measured using a radioimmunoassay or an enzyme-linked immunosorbent assay (ELISA). **The American College of Obstetricians and Gynecologists (ACOG) recommends measuring serum bile acids levels at least twice a week in pregnant women with cholestasis**.

**Why Each Wrong Option is Incorrect**
**Option A:** Serum bilirubin levels are not the primary investigation for cholestasis of pregnancy. While bilirubin levels may be mildly elevated, they do not accurately reflect the degree of bile flow impairment.

**Option C:** Serum alkaline phosphatase levels can be elevated in cholestasis of pregnancy, but they are not specific for diagnosing the condition. Alkaline phosphatase is an enzyme found in various tissues, including bone and liver, and its elevation can be caused by multiple factors.

**Option D:** Serum glutathione transferase levels are not a routine investigation for cholestasis of pregnancy. Glutathione transferase is an enzyme involved in detoxification processes, and its measurement is not directly related to bile flow or cholestasis.

**Clinical Pearl / High-Yield Fact**
It's essential to note that cholestasis of pregnancy can be a severe condition, and prompt diagnosis and management are crucial to prevent fetal distress and other complications. **Monitoring serum bile acids levels regularly can help identify women who require close observation and timely delivery**.

**Correct Answer:**
✓ Correct Answer: B. Serum bile acids levels