A 67-year-old man is recovering on the fourth day after an inferior myocardial infarction treated with primary percutaneous coronary intervention. He suddenly develops severe breathlessness, hypotension and frothy sputum; a new pansystolic murmur is loudest at the apex and radiates towards the left axilla. There is no raised jugular venous pressure or muffled heart sounds. Which one of the following is the most likely diagnosis?