What is the most likely diagnosis for a 33-year-old man presenting with pain, blurred vision, and poor color vision in one eye?

Left Optic Neuritis

The most likely diagnosis is left optic neuritis, given the classic triad of unilateral eye pain, subacute vision loss, and impaired color vision ("washed out" appearance) in a young adult. 1

Clinical Presentation Matches Optic Neuritis

The patient's presentation is textbook for optic neuritis:

• Pain with eye movement is a hallmark feature, occurring in the majority of cases due to inflammation of the optic nerve sheath 2, 3
• Subacute vision loss developing over hours to days (in this case, throughout one day) is characteristic, with vision loss typically occurring over 1-10 days 2, 4
• Poor color vision with colors appearing "washed out" is a key distinguishing feature of optic neuritis, as color vision impairment is disproportionate to visual acuity loss 2, 3
• Age 33 years fits the typical demographic, as optic neuritis is the most common cause of decreased vision from optic nerve dysfunction in patients aged 20-40 years 2, 3

Why Other Diagnoses Are Less Likely

Retinal infarction (central retinal artery occlusion) presents differently:

• Sudden, painless monocular vision loss is the hallmark—not painful vision loss 1
• Impaired color vision is proportional to visual acuity, not disproportionately affected 1
• Typically occurs in older patients with vascular risk factors, not a 33-year-old 1

Migraine visual aura is excluded by:

• Auras are transient (lasting 5-60 minutes), not persistent throughout an entire day 3
• Auras are painless during the visual symptoms, with headache following afterward 3
• No eye pain with movement occurs in migraine 3

Right occipital infarction would cause:

• Painless vision loss 3
• Left homonymous hemianopia (affecting both eyes), not monocular vision loss 3
• No eye pain or color vision changes 3

Expected Examination Findings

On examination, you should look for:

• Relative afferent pupillary defect (RAPD) in the left eye—the direct pupillary light reflex will be weaker in the affected eye compared to the consensual response 2, 3
• Optic disc appearance: One-third of patients have mild disc edema, while two-thirds have a normal-appearing disc initially (retrobulbar neuritis) 3, 4

Immediate Next Steps

MRI of the orbits and brain without and with contrast is the primary imaging study indicated 1:

• Evaluates for abnormal enhancement and signal changes within the optic nerve 1
• Assesses for intracranial demyelinating lesions, which predict subsequent development of multiple sclerosis in 50% of cases 1, 3

The diagnosis is clinical, based on the constellation of symptoms and signs described above 3, 4. MRI confirms the diagnosis and stratifies risk for multiple sclerosis 1, 3.

Critical Pitfall to Avoid

Do not confuse this with anterior ischemic optic neuropathy (AION), which can also cause vision loss but:

• Is painless 1
• Occurs in older patients (typically >50 years) 1
• Has sudden onset (not subacute) 1
• Shows altitudinal visual field defects rather than central scotomas 1