A newborn is delivered at 37 weeks gestation after an uncomplicated pregnancy. Immediately after birth, the infant is noted to have severe midline facial abnormalities, including a single central eye (cyclopia) and a proboscis (a tubular, nose-like structure) above the eye. The infant also exhibits microcephaly (small head size) and hypotonia (low muscle tone). Apgar scores are low, and the baby requires respiratory support shortly after delivery. Imaging studies, including cranial ultrasound and MRI, reveal alobar holoprosencephaly, a severe brain malformation where the forebrain fails to divide into two hemispheres, resulting in a single-lobed brain structure. The infant also has other congenital anomalies, such as cleft lip and palate, polydactyly (extra fingers or toes), and congenital heart defects (e.g., ventricular septal defect). Which chromosomal abnormality is most commonly associated with this condition?
