A 31-year-old woman with chronic hypertension was taking labetalol before conception. At 30 weeks, her blood pressure has risen from 132/82 to 154/98 mmHg. A urine protein:creatinine ratio has increased from 12 to 68 mg/mmol (less than 30), and her platelet count has fallen from 186 to 124 × 10^9/L (150–400). She has no fever, rash or joint symptoms, and fetal growth and liquor volume are normal. Which one of the following is the most likely diagnosis?