Final answer:
The likely diagnosis for this patient is multiple sclerosis (MS).
Step-by-step explanation:
The diagnosis for this patient is likely multiple sclerosis (MS). The painless vision loss in the left eye with an afferent papillary defect suggests optic neuritis, which is commonly seen in MS. The MRI findings of T2 hyperintensities in the white matter periventricularly are also consistent with MS. Additionally, the absence of corpus callosum lesions and lack of enhancement with gadolinium further support the diagnosis.