A 62-year-old patient presents with a unilateral pupil size difference. Which diagnostic test is most likely to identify the cause?

A 62-year-old patient presents with a unilateral pupil size difference. Which diagnostic test is most likely to identify the cause?

• A. CBC with differential
• B. Skull X-rays
• C. MRI with contrast
• D. Serum electrolytes

Answer: (C)

The essential idea is that a unilateral pupil dilation raises suspicion for a focal structural problem affecting the oculomotor nerve, often due to a compressive lesion such as an intracranial aneurysm. MRI with contrast is the best test to identify these causes because it gives detailed images of soft tissues, the cranial nerves, and nearby vessels. The contrast helps highlight aneurysms, tumors, or inflammatory processes that could be pressing on the third nerve and producing a dilated pupil. Why this test fits best: it can directly visualize the nerve pathways and detect small aneurysms or masses that other tests might miss. This is crucial because a pupil-involving third-nerve palsy can indicate a potentially life-threatening aneurysm that requires urgent evaluation. Why the other tests are less helpful: a CBC with differential looks for systemic infection or hematologic issues but would not reveal a compressive brain lesion. Skull X-rays offer little information about intracranial soft tissues or vascular abnormalities. Serum electrolytes don’t address focal neurological pathology and won’t explain new anisocoria.

The essential idea is that a unilateral pupil dilation raises suspicion for a focal structural problem affecting the oculomotor nerve, often due to a compressive lesion such as an intracranial aneurysm. MRI with contrast is the best test to identify these causes because it gives detailed images of soft tissues, the cranial nerves, and nearby vessels. The contrast helps highlight aneurysms, tumors, or inflammatory processes that could be pressing on the third nerve and producing a dilated pupil.

Why this test fits best: it can directly visualize the nerve pathways and detect small aneurysms or masses that other tests might miss. This is crucial because a pupil-involving third-nerve palsy can indicate a potentially life-threatening aneurysm that requires urgent evaluation.

Why the other tests are less helpful: a CBC with differential looks for systemic infection or hematologic issues but would not reveal a compressive brain lesion. Skull X-rays offer little information about intracranial soft tissues or vascular abnormalities. Serum electrolytes don’t address focal neurological pathology and won’t explain new anisocoria.