Differential Diagnosis

• Single most likely diagnosis
  • Encephalitis: The patient's symptoms, including personality changes, confusion, weak right leg, speech impairment, and low-grade fever, along with the brain MRI showing left temporal lobe edema and cerebrospinal fluid analysis with an elevated number of red blood cells, are highly suggestive of encephalitis, which is an inflammation of the brain tissue.
• Other Likely diagnoses
  • Meningitis: Although the absence of nuchal rigidity and negative Gram stain make it less likely, the presence of low-grade fever and elevated red blood cells in the cerebrospinal fluid could still suggest a form of meningitis, particularly if it's viral or if the bacterial meningitis is partially treated.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Meningitis (as mentioned, due to potential for severe consequences if not treated promptly)
  • Myelitis: Although the patient's symptoms are more suggestive of brain involvement, myelitis (inflammation of the spinal cord) could potentially cause some of the neurological deficits seen, and it's crucial not to miss this diagnosis due to its potential for severe and permanent neurological damage.
• Rare diagnoses
  • Frontotemporal dementia: This is a less likely diagnosis given the acute onset of symptoms and the presence of fever and edema on MRI, which are not typical for frontotemporal dementia.
  • Sydenham chorea: This condition is associated with rheumatic fever and typically presents with chorea (involuntary movements) without the significant cognitive and focal neurological deficits seen in this patient, making it an unlikely diagnosis.
  • Other rare diagnoses could include brain abscess, cerebral vasculitis, or certain metabolic disorders, but these would be less likely given the information provided and would typically require additional specific findings or tests for consideration.