A 4-month-old male infant presents with infantile spasms and developmental regression. Neuroimaging reveals a smooth cerebral cortex with absent gyri posteriorly and reduced gyri anteriorly, thickened cortical ribbon, and ventriculomegaly. The clinical and radiological features are consistent with Miller-Dieker syndrome. The genetic deletion marked **A** in the diagram (17p13.3) encompasses the PAFAH1B1 (LIS1) gene. Which of the following best explains the pathophysiological basis of the cortical malformation in this patient?