**Core Concept**
Down's syndrome is a genetic disorder caused by the presence of an extra copy of chromosome 21 (trisomy 21). This condition is associated with various physical and cognitive abnormalities, as well as an increased risk of certain medical complications.

**Why the Correct Answer is Right**
Down's syndrome is characterized by an increased risk of congenital heart defects, particularly atrioventricular septal defects, and other cardiac anomalies. Individuals with Down's syndrome are also more prone to leukemia, particularly acute lymphoblastic leukemia (ALL). Additionally, Down's syndrome is associated with an increased risk of Alzheimer's disease, with many individuals developing dementia in their 40s and 50s.

**Why Each Wrong Option is Incorrect**
**Option A:** While some children with Down's syndrome may experience hearing loss, it is not a universal association and is not as strongly linked to the condition as the other options.
**Option B:** Individuals with Down's syndrome have a higher risk of thyroid dysfunction, including hypothyroidism, but this is not as directly associated with the condition as the other options.
**Option C:** Down's syndrome is not typically associated with increased muscle mass or strength, although some individuals may experience muscle weakness or hypotonia.

**Clinical Pearl / High-Yield Fact**
It's essential to recognize that individuals with Down's syndrome have a unique set of medical needs and require regular monitoring for potential complications, such as cardiac defects and thyroid dysfunction.

**Correct Answer:** D