A 6-month-old boy presents with brief, symmetric flexor spasms of the neck, trunk, and limbs occurring in clusters upon awakening. His parents report developmental arrest over the past 2 months. An EEG is performed, and the pattern marked **A** in the diagram is observed—chaotic, high-voltage (>200 microvolts), disorganized background with multifocal asynchronous spikes and sharp waves intermixed with irregular slow waves, and no normal background rhythm. Which of the following is the most likely diagnosis?