Background: Pulmonary artery aneurysm (PAA) is an exceedingly rare but life-threatening vascular anomaly, typically defined by a diameter exceeding 40 mm in females or 1.5 times the upper limit of normal [1,2]. While Behçet's disease (BD) classically induces distal PAAs via vasculitis, congenital shunts predominantly cause proximal PAAs due to chronic hemodynamic stress [1,2].

Case Presentation: A 53-year-old woman with an established diagnosis of BD and an uncorrected 15 mm atrial septal defect (ASD) presented in July 2024 with severe dyspnea and chest pain. Her main pulmonary artery measured 44 mm. Because her pulmonary vascular resistance (PVR) exceeded 5 Wood units, surgical ASD closure was contraindicated. She was managed medically with macitentan, spironolactone, and furosemide, which significantly improved her functional capacity. She was subsequently lost to follow-up. In August 2026, she returned with recurrent symptoms. Despite stable pulmonary hemodynamics and an inactive inflammatory state (C-reactive protein 2.49 mg/L), imaging revealed a staggering anatomical expansion of the PAA to 84.2 mm. Recognizing the extreme risk of rupture associated with aneurysms >55 mm or those exhibiting rapid growth [1,7], she was urgently referred for surgical graft replacement.

Conclusion: The striking coexistence of ASD-induced PAH and inactive BD points toward a "dual-hit" pathophysiology. Chronic hemodynamic shear stress synergizes with the underlying inflammatory fragility previously established by BD, culminating in extraordinary central aneurysm formation. This case critically highlights that anatomical aneurysmal expansion can progress rapidly despite clinical and hemodynamic stabilization on pulmonary vasodilator therapy.