Background <p>Major aortopulmonary collateral arteries (MAPCAs) are anomalous systemic-to-pulmonary vessels classically associated with complex congenital heart disease. In adults with structurally normal hearts, however, anomalous systemic-to-pulmonary arterial connections represent a broader diagnostic spectrum that includes MAPCA-like vessels, bronchial or non-bronchial systemic arterial collaterals, and isolated systemic arterial supply to normal lung (ISSNL). Aneurysmal degeneration in this setting is uncommon and clinically important because of the risk of rupture or hemorrhage.</p> Case presentation <p>We report a 48-year-old man presenting with acute non-exertional chest pain. Computed tomography angiography (CTA) demonstrated a 2.6-cm saccular aneurysm arising from a tortuous systemic artery originating from the proximal descending thoracic aorta and extending toward the left lower lobe pulmonary arterial circulation. CTA delineated the vascular origin, course, aneurysmal morphology, and absence of associated congenital cardiac abnormalities. Digital subtraction angiography confirmed a single systemic-to-pulmonary arterial communication. The patient underwent thoracic endovascular aortic repair with adjunctive coil embolization, resulting in complete aneurysm exclusion and symptom resolution.</p> Conclusions <p>This case highlights an aneurysmal systemic-to-pulmonary arterial connection with MAPCA-like features in a structurally normal adult. The case emphasizes the importance of CTA in differentiating MAPCA-like vessels from bronchial artery aneurysm, non-bronchial systemic collaterals, and ISSNL, and demonstrates the effectiveness of endovascular therapy in excluding a potentially high-risk aneurysmal vascular lesion.</p>