A 3-year-old female presents with sudden difficulty walking and has a past infection. What is the most likely diagnosis based on her symptoms?

The most likely diagnosis for a 3-year-old female presenting with sudden difficulty walking following a past infection is post-infectious cerebellitis. This condition often occurs after a viral infection, particularly common childhood illnesses such as varicella (chickenpox) or other viral illnesses like influenza.

In post-infectious cerebellitis, the immune response triggered by the previous infection leads to inflammation in the cerebellum, which is the area of the brain responsible for coordinating movement and balance. This inflammation can manifest as ataxia, which is characterized by unsteady gait, difficulty with balance, and various motor coordination challenges. The timing of the neurological symptoms occurring after a recent infection strongly supports this diagnosis.

While other options like acute cerebellar ataxia also relate to coordination issues, post-infectious cerebellitis is specifically indicated by the patient's past infection history and acute presentation. Viral encephalitis and Guillain-Barré Syndrome involve different mechanisms and symptoms, making them less likely in this scenario.