MRCP2-2568

A 39-year-old man presents to the medical outpatient clinic with complaints of increasing breathlessness, particularly on exertion. He has also experienced several fainting episodes in the past few months, with associated chest sensations and rapid recovery. His medical history is unremarkable except for childhood treatment with antibiotics for an eye infection. He has been experiencing constipation and is currently taking Senna and Movicol. On examination, he has a displaced apex beat, normal heart sounds, and mild coarse crepitations at both lung bases. What is the most likely diagnosis?