Final answer:
The most likely cause of the symptoms including neck pain, limited neck motion, and decreased strength of extremities, with 3+ DTR of the upper limb, in a 15-month-old patient with Down syndrome is spinal cord impingement due to atlanto-axial joint subluxation.
Step-by-step explanation:
The symptoms described for a 15-month-old patient with Down syndrome, which include neck pain, limited neck motion, and decreased strength of extremities, accompanied by upper limb deep tendon reflexes (DTR) being 3+, are indicative of a possible spinal cord impingement due to atlanto-axial joint subluxation. In individuals with Down syndrome, there is an increased risk for atlanto-axial instability, which can lead to impingement on the spinal cord that may manifest as changes in strength, range of motion, and reflexes.
While upper motor neuron (UMN) signs can present with strong reflexes and increased tone, they generally affect one side of the body more than the other and would not account for the neck pain or the limited neck motion. Similarly, while people with Down syndrome may inherently have increased tone and reflexes, the acute changes the patient is experiencing suggest a more pressing issue than typical manifestations of the condition. Spinal cord impingement caused by atlanto-axial joint subluxation would most likely lead to the clinical signs observed and should be investigated immediately to prevent further neurological damage.