Pulmonary Hypertension: This condition is characterized by elevated blood pressure in the pulmonary arteries, which can lead to dilation of the pulmonary artery. A diameter of 3.6 cm is significantly larger than the normal range, suggesting chronic elevation of pulmonary pressures.

Left-to-Right Shunt: Conditions such as atrial septal defects (ASD) or ventricular septal defects (VSD) can lead to increased flow through the pulmonary artery, causing it to dilate over time.
Chronic Thromboembolic Pulmonary Hypertension (CTEPH): This condition involves chronic blood clots in the pulmonary arteries, which can increase pressure and lead to dilation of the pulmonary artery.
Pulmonary Valve Regurgitation: Significant regurgitation of the pulmonary valve can lead to increased volume load on the pulmonary artery, causing it to dilate.

Pulmonary Embolism (PE): Although a large pulmonary artery diameter is not typically the first sign of a PE, a significant or saddle embolism could potentially cause acute dilation. Missing this diagnosis could be fatal.
Aortic Dissection Involving the Pulmonary Artery: Although rare, an aortic dissection that extends into the pulmonary artery could cause acute dilation and is a medical emergency.

Arteriovenous Malformation (AVM): A large AVM involving the pulmonary artery could potentially cause dilation due to increased flow, but this would be an uncommon cause.
Pulmonary Artery Aneurysm: Aneurysms of the pulmonary artery are rare and could present with dilation, often associated with other conditions such as Behçet's disease or congenital heart disease.
Vasculitis Involving the Pulmonary Artery: Conditions like Takayasu arteritis or giant cell arteritis can involve the pulmonary arteries, leading to dilation, but these are rare and typically present with other systemic symptoms.