A 32-year-old woman presents with fever, myalgia, headache, photophobia, and neck stiffness. What is the most likely causative organism?

The presentation of fever, myalgia, headache, photophobia, and neck stiffness in a young adult is highly suggestive of meningitis, particularly acute meningitis. Among the pathogens commonly responsible for this condition, Neisseria meningitidis is a leading cause, especially in this demographic.

Neisseria meningitidis, also known as meningococcus, is a bacterium that is a well-known cause of bacterial meningitis, particularly in children and young adults. It is characterized by rapid onset and can lead to severe illness, including the risk of systemic meningococcemia, which can cause septicaemia, purpura, and possibly death if left untreated. The classic triad of symptoms in meningitis includes fever, neck stiffness, and altered mental status, but photophobia and headache are also key indicators suggesting inflammation of the meninges.

The other listed organisms are associated with meningitis but have different epidemiological profiles and clinical presentations. Enterovirus is more commonly associated with viral meningitis, which often has a more benign course compared to bacterial meningitis. Listeria monocytogenes typically affects neonates, pregnant women, and the immunocompromised, often presenting with a different clinical picture. Mycobacterium tuberculosis is linked