A 67-year-old man develops sudden-onset severe headache ('thunderclap'), photophobia, and neck stiffness. CT head is normal. Lumbar puncture shows xanthochromia with 1200 RBCs/mm³ (same count in tubes 1 and 4). The most likely diagnosis is:

• A Bacterial meningitis
• B Aneurysmal subarachnoid hemorrhage ✓
• C Traumatic tap from lumbar puncture
• D Cerebral venous sinus thrombosis

Explanation

Xanthochromia (yellowish discoloration of CSF due to oxyhemoglobin and bilirubin) with a consistent RBC count across tubes 1 and 4 indicates true subarachnoid blood, not a traumatic tap (where RBC count decreases from tube 1 to tube 4). The thunderclap headache and normal CT (sensitivity ~98% within 6 hours, drops to 93% at 24 hours) with xanthochromic CSF confirms subarachnoid hemorrhage — most commonly from ruptured berry aneurysm. CT angiography is the next step to identify the aneurysm.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.