Bacterial Meningitis: This is the most likely diagnosis given the symptoms of fever, headache, stiffness of the neck (nuchal rigidity), and altered mental status. These symptoms are classic for meningitis, which is an inflammation of the protective membranes covering the brain and spinal cord, known as the meninges. The most common causative organisms in this age group are Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae type b (though the latter is less common in vaccinated populations).

Viral Meningitis: This condition presents similarly to bacterial meningitis but is typically less severe and caused by viruses such as enteroviruses, herpes simplex virus, and arboviruses. The symptoms can be similar, but the onset might be more gradual, and the patient might not appear as systemically ill as in bacterial meningitis.
Encephalitis: This is an inflammation of the brain tissue itself, often caused by viral infections. Symptoms can include fever, headache, confusion, and altered mental status, similar to meningitis, but encephalitis may also present with seizures and focal neurological deficits.

Subarachnoid Hemorrhage: Although not infectious, this condition can present with sudden onset of headache and altered mental status. It is crucial to consider this diagnosis because it requires immediate intervention to prevent further brain damage.
Brain Abscess: A localized infection within the brain that can cause fever, headache, and altered mental status, along with focal neurological signs depending on the location of the abscess.
Tuberculous Meningitis: Caused by Mycobacterium tuberculosis, this form of meningitis has a more insidious onset and can present with chronic symptoms of headache, fever, and altered mental status, along with signs of tuberculosis elsewhere in the body.

Fungal Meningitis: More common in immunocompromised patients, fungal meningitis (e.g., cryptococcal meningitis) can present with similar symptoms to bacterial meningitis but often has a more subacute onset.
Rickettsial Infections: Such as Rocky Mountain spotted fever, which can cause fever, headache, and altered mental status, but typically also includes a rash and exposure history.
Lyme Meningitis: Caused by Borrelia burgdorferi, this condition can present with symptoms of meningitis, especially in endemic areas and during certain times of the year.

Cerebrospinal Fluid (CSF) Analysis: This is crucial for diagnosing meningitis. The fluid is obtained through a lumbar puncture and analyzed for:
- Appearance: Turbid or cloudy CSF suggests infection.
- Glucose and Protein Levels: Low glucose and high protein levels are indicative of bacterial meningitis.
- Cell Count: Elevated white blood cell count, with a predominance of neutrophils in bacterial meningitis and lymphocytes in viral or tuberculous meningitis.
- Gram Stain: Direct staining of CSF to identify bacteria.
- Culture: CSF is cultured on appropriate media (e.g., blood agar, chocolate agar) to isolate the causative organism.
- PCR (Polymerase Chain Reaction): For detecting viral or bacterial DNA in the CSF.
Blood Cultures: To identify bacteremia.
Imaging Studies: CT or MRI of the head may be performed to rule out other causes of symptoms, such as abscesses or hemorrhages, especially before performing a lumbar puncture if there are concerns about increased intracranial pressure.

Gram Stain: For initial identification of bacteria as Gram-positive or Gram-negative.
Blood Agar: Supports the growth of Streptococcus pneumoniae, Haemophilus influenzae, and other fastidious organisms.
Chocolate Agar: Used for Haemophilus and Neisseria species.
Thayer-Martin Agar: Selective for Neisseria meningitidis.
Sabouraud Agar: For fungal cultures.
Lowenstein-Jensen Medium: For Mycobacterium tuberculosis.