Transient ischemic attack

A 68-year-old woman is brought to the emergency department following a fall earlier this morning. She explains that she got out of bed a bit quicker than she normally does, and felt the room spinning before her knees buckled and she fell to the floor. She continued to feel the room spinning for around 10 minutes before her symptoms resolved. On examination, there is a mild laceration above her right eye. She is otherwise stable and has no evidence of any orthopedic trauma. The Dix-Hallpike maneuver and supine roll test are both negative, and there is no nystagmus or problems with hearing. Her past medical history includes coronary artery disease, hypertension, and type 2 diabetes mellitus. Which of the following is the most likely cause of this woman's fall?