Complex sentence: Athetosis is a condition primarily caused by the marbling, or degeneration of the basal ganglia most commonly due to complications at birth with rare cases in which damage to this structure may also arise later in life due to stroke or trauma.
Split into simpler sentences: Athetosis is a symptom primarily caused by the marbling, or degeneration of the basal ganglia. This degeneration is most commonly caused by complications at birth in addition to rare cases in which the damage may also arise later in life due to stroke or trauma.