**Core Concept:** Down's syndrome is a genetic disorder caused by the presence of an extra copy of chromosome 21. It is associated with developmental and physiological abnormalities, including mental retardation.

**Why the Correct Answer is Right:** Down's syndrome results from the presence of an extra copy of chromosome 21, which leads to overexpression of certain genes. This overexpression disrupts normal cellular processes, contributing to the various abnormalities observed in the syndrome. Mental retardation is a common feature of Down's syndrome, resulting from the disruption of normal cognitive development due to the gene overexpression.

**Why Each Wrong Option is Incorrect:**

A. **Option A:** Short stature is often observed in Down's syndrome patients due to the disruption of growth hormone synthesis and action. This option is incorrect because mental retardation does not negate the possibility of short stature in Down's syndrome patients.

B. **Option B:** Increased risk of leukemia is a known complication in Down's syndrome individuals due to the disruption of cellular differentiation. This option is incorrect because mental retardation does not negate the increased risk of leukemia in Down's syndrome patients.

C. **Option C:** Increased risk of Alzheimer's disease is a well-known complication in Down's syndrome patients due to the disruption of amyloid precursor protein processing. This option is incorrect because mental retardation does not negate the increased risk of Alzheimer's disease in Down's syndrome patients.

D. **Option D:** Increased risk of congenital heart defects is a known complication in Down's syndrome patients due to the disruption of cardiac development. This option is incorrect because mental retardation does not negate the increased risk of congenital heart defects in Down's syndrome patients.

**Clinical Pearl:** Down's syndrome is a crucial example of the impact of genetic mutations on cellular processes, leading to a range of developmental and physiological abnormalities. While mental retardation is a significant feature, it does not negate the co-occurrence of other complications, such as increased risk of leukemia, Alzheimer's disease, congenital heart defects, and short stature. This highlights the importance of comprehensive evaluation and management of Down's syndrome patients to minimize morbidity and mortality.