This man complains of breathlessness which improves when he lies flat. The reason, in this case, is due to an intra-pulmonary shunt. Alpha 1 AT deficiency is a lower lobe predominant disease which means that when standing, the blood gravitates towards the affected lower lobes resulting in poor gas exchange due to a ventilation-perfusion mismatch. On lying flat, the pulmonary circulation is not affected by gravity as much and is able to circulate and exchange gases within the mid and upper lobes of the lung to a greater extent, reducing the ventilation-perfusion mismatch. 

Alpha-1-antitrypsin deficiency is the correct answer as it explains the breathlessness on lying down most completely.

Guillain barre can cause breathlessness which is relieved by lying down due to autonomic failure but the history is too long.

Langerhans cell histiocytosis typically has an upper lobe predominance meaning symptoms would improve on standing up compared to lying down.

Mesothelioma is a cause of breathlessness due to pleural effusions but this would not improve with lying down.

Ventricular septal defects can cause severe breathlessness, but it is actually atrial septal defects that can vary (and improve) with lying down.