Beginning in adolescence, which condition is associated with progressive instability, ataxia, and inability to walk?

The condition associated with progressive instability, ataxia, and inability to walk beginning in adolescence is olivopontocerebellar atrophy. This disorder is characterized by degeneration of specific areas of the brain, particularly the olivary nuclei, pons, and cerebellum, leading to progressive motor control issues. Ataxia, which refers to uncoordinated movement, is a hallmark symptom, impacting an individual's ability to maintain balance and walk effectively.

In this condition, symptoms typically manifest in late adolescence or early adulthood, making it distinctive from the other options listed. While cystic fibrosis primarily affects the lungs and digestive system, retinitis pigmentosa mainly leads to vision loss, and rheumatic fever involves inflammatory responses to infections that can affect the heart and joints, none of these involve the specific motor control issues that occur with olivopontocerebellar atrophy. Therefore, it is this degenerative condition that aligns with the symptoms of instability and progressive ataxia beginning in adolescence.