Optic Nerve Coloboma Congenital

A 6-year-old boy presents with poor vision in the left eye detected during school screening. Best-corrected visual acuity is 6/60 in the left eye with −8.0 D myopia. Dilated fundus examination reveals a large, well-circumscribed, white, bowl-shaped excavation of the inferior optic disc extending into the adjacent inferior retina and choroid, with central retinal vessels emerging from the upper rim and diving into the excavation. The structure marked **A** in the diagram represents this finding. Which embryological event is responsible for the formation of this optic disc anomaly?