A 2-month-old male infant born to non-consanguineous parents presents with a high-pitched, monochromatic, feline-like cry, severe hypotonia, microcephaly, and widely spaced eyes. Karyotype analysis is performed. The structure marked **A** in the diagram shows a terminal deletion of the short arm of chromosome 5. Which of the following clinical features is MOST PATHOGNOMONIC for the chromosomal abnormality represented by this deletion and typically prompts initial diagnostic suspicion in the neonatal period?