MRCP-0056

A 42 yr. old man presented with palpitations and shortness of breath on exertion. On examination his lungs were clear and heart sounds were normal. There was a left sided parasternal heave. His electrocardiogram (ECG) revealed atrial fibrillation with right axis deviation. Echocardiography showed dilated right heart chambers. Left and right heart catheterisation study revealed the following: Inferior vena cava Oxygen saturations 63 %, Superior vena cava Oxygen saturations 59 %, Right atrium Oxygen saturations 77 %, Right ventricle Oxygen saturations 78 %, Pulmonary artery Oxygen saturations 82 %, Arterial saturation Oxygen saturations 98 %. What is the most likely diagnosis from the following answers?