Viral Meningitis: This condition is characterized by the presence of mononuclear cells in the cerebrospinal fluid (CSF), which is consistent with the given findings. The presence of 1+ polys (polymorphonuclear cells) can sometimes be seen in the early stages of viral meningitis.

Bacterial Meningitis (Early Stage): Although bacterial meningitis typically presents with a higher number of polys, early in the course of the disease, the CSF may show a mix of mononuclear and polymorphonuclear cells.
Partially Treated Bacterial Meningitis: If a patient has been partially treated with antibiotics before the lumbar puncture, the CSF findings might show a decrease in the number of polys, with an increase in mononuclear cells.
Fungal or Tubercular Meningitis: These infections can present with a mononuclear cell predominance in the CSF, although the clinical context and other CSF parameters (like low glucose and high protein) would also be important for diagnosis.

Subarachnoid Hemorrhage: Although the CSF cell count is not diagnostic, the presence of blood in the CSF can sometimes lead to an increase in white blood cell count, including mononuclear cells and polys. It's crucial to check for xanthochromia and blood in the CSF.
Central Nervous System (CNS) Lymphoma: This condition can present with mononuclear cells in the CSF, and it's critical not to miss it due to its severe implications.

CNS Sarcoidosis: This condition can cause an increase in mononuclear cells in the CSF, but it's less common and usually associated with other systemic symptoms of sarcoidosis.
CNS Vasculitis: This is a rare condition that can cause inflammation in the CNS, leading to an increase in mononuclear cells in the CSF, among other findings.