25-year-old male presents to the emergency department with his friend in a state of unconsciousness. His friend said he had a “fit” 20 minutes ago and another one on the way to the emergency room. His friend informs you that the patient is a known epileptic and you find a pack of phenytoin on him. You notice that he also smells of alcohol. He has a blood pressure of 170/95 mmHg and temperature 37.9°C. On examination there is a quiet systolic murmur, though it is difficult to fully characterize. Which of the following investigations will be most useful at this stage?