What is a clinical sign of Down's atlantoaxial instability?

Down's syndrome, or trisomy 21, is associated with various musculoskeletal and neurological abnormalities, one of which includes atlantoaxial instability. This condition arises from an abnormal space between the first two cervical vertebrae (the atlas and axis), leading to potential compression of the cervical spinal cord.

The presence of upper motor neuron signs indicates involvement of pathways that control voluntary movements and reflexes. These signs may include weakness, increased muscle tone, and exaggerated deep tendon reflexes. When compression occurs due to atlantoaxial instability, it can lead to symptoms such as incontinence and dizziness due to the disruption of normal neural pathways. The compression can affect brainstem function or the upper spinal cord, resulting in a combination of neurological deficits that reflect upper motor neuron involvement.

Considering these factors, the occurrence of upper motor neuron signs alongside incontinence and dizziness reflects a clear manifestation of spinal cord compression due to atlantoaxial instability, making this the correct answer regarding clinical signs associated with this condition in individuals with Down syndrome.