West Syndrome Hypsarrhythmia

An 8-month-old boy presents with clusters of brief, symmetric flexor spasms occurring predominantly on arousal. His parents report developmental arrest over the past 2 months. EEG shows the pattern marked **A** in the diagram. Which of the following is the MOST COMMON identifiable etiology of this condition in this patient?

A 6-month-old boy presents with a 3-week history of clusters of brief symmetric spasms involving sudden flexion of the neck, trunk, and arms, occurring predominantly on awakening from sleep. The child has lost previously acquired developmental milestones including social smiling and head control. MRI brain shows cystic encephalomalacia in bilateral watershed zones consistent with prior hypoxic-ischemic injury. An interictal EEG during drowsiness shows the pattern marked **A** in the diagram. Based on this EEG finding and clinical presentation, what is the most likely diagnosis?