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A positive sulcus sign is indicative of inferior glenohumeral instability in the shoulder. When the arm is pulled downward, and a noticeable gap or sulcus appears between the acromion and the humeral head, it suggests that the shoulder joint is not maintaining its structural integrity. This is commonly seen in conditions involving instability of the shoulder rather than the other conditions listed.

In the context of the options provided, a cerebrovascular accident typically does not manifest with a sulcus sign, as it primarily affects neurological function rather than joint stability. Early-onset Alzheimer's disease relates primarily to cognitive decline and is not associated with physical joint instability. A rupture of the biceps tendon may affect the shoulder but is not specifically indicated by a sulcus sign. Lastly, Duchenne muscular dystrophy, which involves muscle weakness and atrophy, may impact the shoulder but does not characteristically produce a sulcus sign. Therefore, the presence of a positive sulcus sign is most consistent with shoulder instability rather than the other medical conditions listed.