Pulmonary Arterial Hypertension (PAH): This condition is characterized by elevated blood pressure in the pulmonary arteries, which can lead to dilation of the main pulmonary trunk. PAH can present with normal pulmonary function tests and a normal ejection fraction, making it a likely diagnosis given the provided symptoms.

Chronic Thromboembolic Pulmonary Hypertension (CTEPH): This condition occurs when blood clots form in the pulmonary arteries, leading to increased pressure and potential dilation of the main pulmonary trunk. It's a possible diagnosis, especially if there's a history of thromboembolic events.
Pulmonary Valve Regurgitation: Significant regurgitation can lead to dilation of the main pulmonary trunk due to the increased volume load. Hypertension could be secondary to other factors or related to the pulmonary valve disease itself.

Pulmonary Embolism (Large or Multiple): Although less likely if pulmonary function tests are normal, a large or multiple pulmonary emboli could cause acute dilation of the main pulmonary trunk and hypertension. Missing this diagnosis could be fatal.
Aortic Disease (e.g., Aortic Aneurysm or Dissection): While primarily affecting the aorta, significant aortic disease can indirectly affect pulmonary artery pressure and anatomy, especially if there's involvement of the aortic root or if it leads to secondary pulmonary hypertension.

Arteriovenous Malformations: Rare congenital or acquired conditions that could lead to abnormal blood flow and pressure within the pulmonary arteries, potentially causing dilation of the main pulmonary trunk.
Pulmonary Capillary Hemangiomatosis: A rare condition characterized by abnormal capillary proliferation within the lung, leading to pulmonary hypertension and potentially dilation of the pulmonary trunk.
Pulmonary Veno-Occlusive Disease: A rare form of pulmonary hypertension caused by occlusion of the small pulmonary veins, which could lead to the described symptoms, although it's much less common.