A 74-year-old male ex-comedian can no longer perform at smoky open- microphone nights due to shortness of breath. He is coughing up frothy white sputum and has recently noticed small amounts of blood in it. On examination, his chest demonstrates diffuse crackles on inspiration through which you can just discern a mid-diastolic murmur and a loud first heart sound. His chest X-ray confirms pulmonary oedema. What is the most likely underlying cause for his symptoms?