The presence of a single demyelinating focus with signs of the blood-brain barrier (BBB) disruption, revealed during magnetic resonance imaging (MRI), in the absence of clinical symptoms, is most often an accidental finding and requires careful follow-up and anamnesis clarification for diagnosis. Such a focus can be a sign of multiple sclerosis onset, be a symptom of acute disseminated encephalomyelitis, inflammatory demyelinating pseudotumor, brain tumor, encephalitis, etc. Difficulties of differential diagnosis in a radiologically isolated syndrome are reflected in the presented clinical case of a 26-year-old woman who had recovered from COVID-19, in whom an MRI revealed a large single focus in the left frontal lobe with signs of active process.