Final answer:
Chronic kidney disease is the least likely coexisting condition in a 6-year-old boy with Down syndrome facing surgery. Down syndrome more commonly presents with cardiac defects, airway issues, and atlanto-axial instability. CKD is less directly associated with Down syndrome than these other issues.
Step-by-step explanation:
The question involves identifying the coexisting condition that is least likely to be present in a 6-year-old boy with Down syndrome undergoing tonsillectomy and adenoidectomy. Among the options provided, chronic kidney disease (CKD) is the least likely associated condition. Children with Down syndrome commonly present with cardiac defects, airway obstruction, and atlanto-axial instability due to their genetic condition, as Down syndrome results from nondisjunction of chromosome 21 leading to an extra chromosome 21 or a part of it. Subglottic stenosis can also be seen due to airway anomalies. However, while CKD is not completely unrelated to Down syndrome, it is not one of the primary concerns typically associated with this genetic disorder, unlike the cardiac, airway, or musculoskeletal issues mentioned.