Gestational hypertension

A 20-year-old woman, gravida 1 para 0 comes to the clinic at 22 weeks of gestation for a follow-up appointment after she had an elevated blood pressure of 145/95 in the previous appointment. The pregnancy has been uneventful so far, and she has been taking prenatal vitamins daily. Medical history is unremarkable. She has not had any headaches, visual disturbances or urinary symptoms. She does not smoke cigarettes, drink alcohol or use illicit drugs. Repeated blood measurements at home are 147/92 and 146/93.  The patient’s current temperature is 37.0°C (98.6°F), pulse is 100/min, and blood pressure is 144/93 mmHg. On physical examination, cardiovascular and lung examination are unremarkable. Uterus size is consistent with 22 weeks, and fetal vital signs on nonstress test are reassuring. Urinary protein excretion on 24-hour urine is minimal, and serum creatinine concentration is 1.1 mg/dL. Which of the following is the most likely diagnosis?