A 68-year-old man is brought in by his wife after his third collapse in a month. Each time he has been sitting watching television, has suddenly gone grey and slumped without any warning, and has come round within a minute, fully alert and able to continue the conversation. He denies palpitations or chest pain and has never collapsed on exertion. He takes amlodipine for hypertension. Examination is unremarkable, with no murmur and no postural fall in blood pressure. His electrocardiogram in the clinic shows sinus rhythm at 58/min, a PR interval of 0.24 seconds, a QRS duration of 0.14 seconds with an rSR' pattern in lead V1, and a frontal-plane axis of −45 degrees.

Which one of the following is the most likely cause of his collapses?

Keep going — a set of ten like this one, one at a time.

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