What test is contraindicated in patients with focal neurologic signs or symptoms?

Lumbar puncture is contraindicated in patients with focal neurologic signs or symptoms due to the risk of cerebral herniation. When a patient has focal neurologic deficits, it may indicate increased intracranial pressure or a mass effect—conditions that could be exacerbated by the procedure. Performing a lumbar puncture in such situations can lead to a rapid decrease in pressure in the spinal canal compared to the pressure in the cranial cavity, potentially causing brain structures to herniate through the foramen magnum, which can be life-threatening.

In contrast, the other tests, such as blood tests, CT scans, and electroencephalograms, do not carry the same immediate risks associated with altering intracranial pressure and can provide critical information for diagnosing a variety of conditions without posing a direct risk of herniation. Therefore, it's essential to assess the patient's neurologic status before deciding on a lumbar puncture to avoid serious complications.