In a patient presenting with a headache and neck stiffness, which condition is most likely if the patient has positive nuchal rigidity?

When a patient presents with a headache and neck stiffness, and positive nuchal rigidity is noted, meningitis is the condition most closely associated with these symptoms. Nuchal rigidity refers to resistance or stiffness in the neck when trying to flex it forward, which is a classic sign of meningitis. This condition involves inflammation of the protective membranes covering the brain and spinal cord, which can lead to increased intracranial pressure and irritability of the meninges. Patients often also present with other systemic symptoms such as fever, altered mental status, and potentially rash, depending on the etiology of the meningitis (bacterial or viral).

Other conditions listed can present with headaches and neck stiffness but do not primarily feature nuchal rigidity as a key sign. Encephalitis may cause similar symptoms but usually involves altered mental status more prominently, and while it can have meningeal signs, nuchal rigidity is less commonly emphasized. Subarachnoid hemorrhage can present with a sudden, severe headache (often described as a "thunderclap headache") and neck stiffness due to meningeal irritation, but nuchal rigidity is not as definitive a sign as it is in meningitis. Cervical spine injury can also present with neck