Which of the following is NOT a diagnosis of exclusion?

• A. NAION
• B. AAION
• C. Bell's palsy
• D. IIH

Answer: (B)

The concept being tested is how diagnoses are established: some conditions are identified primarily by positive evidence of a specific disease process, while others are reached by ruling out alternative causes (diagnoses of exclusion).

Arteritic anterior ischemic optic neuropathy is not a diagnosis of exclusion. It is defined by an underlying inflammatory vasculopathy—giant cell arteritis. The diagnosis rests on clinical features (for example, onset in an older patient with sudden vision loss plus signs like scalp tenderness or jaw claudication) and objective tests (elevated ESR/CRP, temporal artery biopsy or appropriate imaging). Because you’re identifying a concrete inflammatory disease that directly explains the optic neuropathy, you don’t rely on excluding other conditions to name it; you actively confirm the presence of giant cell arteritis and treat promptly.

The other conditions listed are typically considered diagnoses of exclusion because they rely on ruling out other causes of the presenting finding: NAION is diagnosed after excluding arteritic AION and other optic neuropathies; Bell’s palsy is diagnosed after excluding stroke, tumors, and other causes of facial weakness; IIH is diagnosed after imaging and investigations rule out mass lesions, venous sinus thrombosis, hydrocephalus, and other causes of papilledema.