A 27-year-old woman, who has been living alone, is found by a friend in a confused condition and is brought to the hospital emergency room. No history of drug ingestion can be elicited from the friend. The general physical examination is unremarkable. The patient is responsive but semistuporous, unable to give a coherent history. The ECG is shown.What is the essential abnormality in the ECG?What abnormal clinical laboratory result might be predicted?What underlying clinical diagnosis is most likely?