A 65 year-old patient presents with severe headache, nausea, and a stiff neck. Which diagnosis is most likely?

The presentation of severe headache, nausea, and a stiff neck in a 65-year-old patient is highly suggestive of bacterial meningitis. This condition is characterized by inflammation of the protective membranes covering the brain and spinal cord, which is often caused by a bacterial infection. The classic symptoms of bacterial meningitis include a sudden onset of a severe headache, altered mental status, fever, and neck stiffness due to meningeal irritation.

Nausea is also a common symptom associated with this condition, stemming from increased intracranial pressure or irritation of the meninges. The combination of these symptoms, along with the patient's age, raises significant concern for bacterial meningitis, particularly since older adults are more susceptible to infections and may have a more severe disease course.

The other options are less likely based on the combination of symptoms presented. While transient ischemic attacks can present with headache and other neurological symptoms, they typically do not cause neck stiffness or nausea as primary symptoms. Migraine headaches can cause severe pain and nausea but would not typically produce neck stiffness. Cryptococcosis is a fungal infection that can also cause meningitis but is less common as an initial diagnosis compared to bacterial meningitis, especially in an acute presentation with these symptoms.

Therefore,