MSFinals-0072

A 75-year-old woman, who is a nursing home resident, presents to the Emergency Department, complaining of a one-week history of a red swollen calf, nausea and ‘ants on her arm’. She is noted to be unsteady on her feet and cannot remember what medications she is on. Observations find her temperature is 38.6 °C, oxygen saturation 98%, blood pressure 90/60 mmHg, heart rate 90 bpm and respiratory rate 20 breaths per minute.
What is the most likely cause of her hypotension?