For a patient exhibiting nuchal rigidity and fever, what would be the most appropriate immediate investigation?

In the scenario presented, where a patient is exhibiting nuchal rigidity and fever, the most appropriate immediate investigation is a lumbar puncture. This procedure allows for the direct analysis of cerebrospinal fluid (CSF), which is crucial in diagnosing conditions such as meningitis, a serious infection that can cause these symptoms.

Nuchal rigidity, which is stiffness of the neck, often indicates irritation of the meninges, and when combined with fever, raises concern for meningitis or encephalitis. A lumbar puncture can help differentiate between viral, bacterial, or other types of meningitis based on the appearance, cell count, and biochemical composition of the CSF. Timely diagnosis is critical because bacterial meningitis, in particular, can be life-threatening.

While investigating such symptoms, other options may provide valuable information but are not as immediately indicative of a central nervous system infection. For instance, a CT scan could be considered to rule out mass lesions or other structural abnormalities before a lumbar puncture in certain circumstances, especially if there are signs of increased intracranial pressure. Blood cultures can be helpful in detecting systemic infections but would not specifically address the suspicion of meningitis without information about the state of the CSF. An X-ray of the