The most appropriate next step is cardiac rhythm monitoring. The patient has an acute infarction in the anterior cerebral artery territory, which is typically the result of artery-to-artery embolic sources or cardiac embolism. This diagnosis is supported by the abrupt termination of the artery seen on magnetic resonance angiography. Until final stroke classification can be confirmed, the patient has a cryptogenic stroke. The patient's vascular imaging did not show extracranial or intracranial internal carotid artery stenosis, which makes cardiac embolism more likely. In several reports, as many as 25% of patients with cryptogenic ischemic stroke have paroxysmal atrial fibrillation on prolonged cardiac monitoring of up to 30 days; patients who have premature atrial contractions and other findings of ectopy on short-term telemetry may be more likely to have this finding. Continued cardiac rhythm monitoring to detect atrial fibrillation is thus advisable in this patient.

Clopidogrel should not be added to this patient's medication regimen because the combination of aspirin and clopidogrel was associated with an increased risk of major hemorrhage without any associated clinical benefit in several clinical trials in the subacute setting.

Dabigatran and other novel anticoagulants have only been approved for stroke prevention in the setting of atrial fibrillation, which has not yet been diagnosed in this patient.

Patent foramen ovale (PFO) closure did not reduce the risk of ischemic stroke more than best medical therapy in patients with cryptogenic stroke in the recently completed CLOSURE I trial. The risk of recurrent stroke in patients with an otherwise isolated PFO, with or without an atrial septal aneurysm, is low in most clinical trials. In older patients, a PFO frequently is an incidental finding and unlikely to be causally related to the infarct.