Primary Optic Atrophy

A 34-year-old woman with relapsing-remitting multiple sclerosis presents with progressive painless vision loss in her left eye over 6 months, following an acute retrobulbar optic neuritis episode 18 months earlier. She reports colour desaturation and reduced brightness (Uhthoff phenomenon). On examination, she has a marked relative afferent pupillary defect in the left eye, severe red desaturation on Ishihara testing (2/15), and a centrocecal scotoma on visual fields. Dilated fundus examination of the left eye reveals the appearance shown as structure **C** in the diagram. Which of the following best characterizes the pathophysiological mechanism underlying the optic disc appearance at **C**?