A 47-year-old woman is evaluated in the emergency department for recurrent attacks of severe holocranial pain that began 2 days ago while she was gardening. She first experienced a rapidly progressive, global, explosive headache that lasted 30 minutes and was associated with photophobia and phonophobia. She had identical symptoms 6 hours ago and additionally had 30 minutes of visual blurring and numbness of the left face and left upper extremity. The patient has attention-deficit disorder for which dextroamphetamine was initiated 2 weeks ago. She has no significant headache history.

On physical examination, blood pressure is 130/90 mm Hg and pulse rate is 86/min. Left homonymous hemianopia is noted.

An MRI of the brain shows bilateral occipital areas of acute infarction.

Cerebrospinal fluid analysis shows 10 erythrocytes, 4 leukocytes, and normal protein and glucose levels.

Four hours after entering the emergency department, the patient has a third abrupt-onset headache with worsening visual blurring. Blood pressure is now 190/115 mm Hg, but physical examination and neuroimaging findings are unchanged.

Which of the following is the most appropriate treatment?