a Marcus Gunn pupil is the same thing as an APD. This finding is suggestive of a problem with the optic nerve. Optic neuritis typically is associated with eye pain behind the eyeball with movement. Vision typically returns in about 4-6 weeks, however, with or without steroid treatment.
Optic neuropathy is another generally term for optic nerve degeneration but not associated iwth inflammation. Tumors compressing on the nerve can do this (something an MRI would pick up). You should have gotten imaging of the area around the eyeball, so the scans should have included the "orbits
You may have something called LHON or Leber's hereditary optic neuropathy. This is a genetic disease inherited through the mother's genes. This usually affects one optic nerve, then the other a few weeks later. The appearance of the nerve is distinct. You may have a varient of this that is not typical. Genetic testing is available.
Optic neuropathy can be due to toxicity or nutritional defiency. Medications such as ethambutol, tobacco, Vitamin B6/B12 defienciency, alcohol. These things would tend to be more bilateral.
Unfortunately, optic neuropathy is a diagnosis that has obvious vision loss, but difficult to find an exact cause. What I mentioned above will likely be considered as possibility. You may undergo a repeat CT scan including the orbits, possible DNA testing, and other blood testing.
Glaucoma would be fairly easy to diagnose in patients with advance field loss. You are also very young for glaucoma. It is unlikely that.
I am unable to give you a clear diagnosis, but hopefully, this answer will give you a framework when considering all the possibilities. Best of luck to you. Please remember to press accept.