uk-drcog MCQ (SBA): Obstetric Cholestasis

A 28-year-old primigravida presents to the GP at 34 weeks of gestation with generalised itching, worse on the palms of her hands and soles of her feet, particularly at night. She has no rash. Her blood pressure is 118/72 mmHg and urinalysis is normal. Which is the single most appropriate initial investigation?