<p>Truncus arteriosus (TA) is a rare conotruncal malformation in which a single arterial trunk supplies the systemic and pulmonary circulations. Severe left ventricular hypoplasia in association with TA is exceptional and creates a complex neonatal circulatory disorder. In a forensic setting, such malformations may be encountered as unexpected neonatal deaths, particularly when antenatal diagnosis is absent. We report the forensic autopsy findings of a term male neonate with a birth weight of 4260&#xa0;g, born to first-cousin parents after a pregnancy without regular antenatal follow-up. Respiratory distress developed on the third day of life, and the infant died on the seventh day despite intensive care. Clinical records described a complex congenital cardiac anomaly with single-atrium/single-ventricle physiology. Autopsy demonstrated Collett-Edwards type II TA, a markedly hypoplastic aorta, a single common atrium consistent with complete atrial septal defect, absence of a ventricular septal defect, severe left ventricular hypoplasia, a bileaflet atrioventricular valve, a tricuspid truncal valve, and a widely patent ductus arteriosus acting as the main conduit to the aortic arch. The coronary arteries originated from the hypoplastic aorta. Histology showed multifocal acute ischemic myocardial injury and hepatic hemorrhagic necrosis. Toxicological analysis did not identify a toxic cause of death. This case illustrates a fatal and exceptionally rare association of type II TA, common atrium, and severe left ventricular hypoplasia. The findings support the value of complete forensic autopsy, histopathology, toxicology, and careful clinicopathological correlation in unexpected neonatal death. The case also highlights the importance of standardized cardiac dissection and photographic documentation in neonatal forensic autopsies involving complex congenital heart disease.</p>