Cri du chat syndrome

A 3-hour-old female neonate is being evaluated in the newborn nursery for hypotonia. The infant was born via vaginal delivery to a 25-year-old mother, who did not have a perinatal follow-up. Family history is non-contributory. The patient’s birth weight is below the 5th percentile, height at the 6th percentile, and head circumference at the 3rd percentile. On examination, the child appears hypotonic, and her cry is high-pitched and cat-like. In addition, she has a moon face, hypertelorism, a flat nose, and a prominent nasal bridge. Which of the following is most likely responsible for this patient’s condition?