In a case of an 18-year-old with a 3-day headache, fever, and signs of Kernig's and Brudzinski's, what is the most likely diagnosis?

The presence of a headache, fever, and positive signs of Kernig's and Brudzinski's in an 18-year-old strongly suggests meningitis, particularly because these symptoms point toward irritation of the meninges, the protective membranes covering the brain and spinal cord.

Bacterial meningitis is more common in young adults and can develop rapidly, leading to severe symptoms. The combination of headache, fever, and the specific signs of meningitis indicates irritation of the central nervous system, which corresponds with an infectious process, making bacterial meningitis the most likely diagnosis in this context. The acute onset of symptoms is typical for bacterial causes.

In contrast, viral meningitis generally presents with similar symptoms but tends to be less severe and often resolves more quickly. Tension headaches and migraines typically do not present with fever or meningismus (the signs of Kernig's and Brudzinski's) and would be less likely in this case due to the accompanying systemic signs. Thus, the clinical presentation aligns most closely with bacterial meningitis, necessitating prompt evaluation and treatment.