**Core Concept**
Down's syndrome, also known as Trisomy 21, is a genetic disorder caused by the presence of an extra copy of chromosome 21. This condition affects various physiological systems, leading to characteristic clinical features and increased susceptibility to certain medical conditions.

**Why the Correct Answer is Right**
Individuals with Down's syndrome are at an increased risk of developing Alzheimer's disease, particularly after the age of 40. The underlying mechanism is related to the overexpression of genes on chromosome 21, including the amyloid precursor protein (APP) gene. The accumulation of amyloid beta peptides, a product of APP processing, is thought to contribute to the pathogenesis of Alzheimer's disease in Down's syndrome.

**Why Each Wrong Option is Incorrect**
**Option A:** Heart defects are a common feature in Down's syndrome, occurring in up to 50% of cases. The increased risk is likely due to the disruption of cardiac development during embryogenesis.
**Option B:** Leukemia, particularly acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML), is a significant concern in children with Down's syndrome. The exact mechanism is unclear, but it's thought to be related to the abnormal immune system and the increased risk of infections.
**Option C:** Gastroesophageal reflux disease (GERD) is not a characteristic feature of Down's syndrome, although some individuals may experience gastrointestinal symptoms. The primary concerns in Down's syndrome are related to cardiac, hematological, and neurological manifestations.

**Clinical Pearl / High-Yield Fact**
Individuals with Down's syndrome are at an increased risk of thyroid dysfunction, particularly hypothyroidism, due to the presence of a duplication of the thyroid-stimulating hormone (TSH) receptor gene on chromosome 21.

**Correct Answer: C. Gastroesophageal reflux disease (GERD)**