A 32-year-old woman at 34 weeks gestation reports two weeks of intense itching that is worse at night and began on her hands and feet. She has no rash, abdominal pain or fever; blood pressure is 118/72 mmHg, platelets are 186 × 10⁹/L (150–400) and glucose is 4.8 mmol/L (3.5–7.8). Alanine aminotransferase is 220 U/L (5–40), bilirubin is 28 micromol/L (3–20), and total serum bile acids are 78 micromol/L (0–10). Which one of the following is the most likely diagnosis?