American Board of Internal Medicine (ABIM) Certification Practice Exam

When suspicion of a subarachnoid hemorrhage is high but noncontrast CT is normal, what test is required?

1. Electroencephalogram

2. Lumbar puncture

3. Magnetic resonance angiography

4. Cerebral angiogram

The correct answer is: Lumbar puncture

When there is a strong suspicion of a subarachnoid hemorrhage (SAH) but the initial noncontrast CT scan does not show any evidence of bleeding, a lumbar puncture is the appropriate next step. This procedure allows for the analysis of cerebrospinal fluid (CSF), which can provide critical information. In cases of SAH, even if the CT is normal, the CSF analysis can reveal the presence of red blood cells and xanthochromia (a yellow discoloration of the CSF due to the breakdown of hemoglobin), both of which indicate prior bleeding into the subarachnoid space. This makes a lumbar puncture a valuable tool for confirming or ruling out SAH when imaging studies are inconclusive. Other options, while potentially useful in different contexts, do not serve as the immediate follow-up test for initial suspicion of SAH after a normal CT. For instance, an electroencephalogram would assess brain activity rather than directly evaluate for bleeding. Magnetic resonance angiography and cerebral angiogram are advanced imaging studies that can provide information about vascular structures and potential aneurysms but are not the first-line diagnostic tools when initial imaging does not show hemorrhage. Thus, a lumbar puncture is the best choice in