**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 is associated with various physical and developmental abnormalities, including intellectual disability, delayed speech, and distinct facial features.

**Why the Correct Answer is Right**
The correct answer will be the option that is not typically associated with Down's syndrome. Down's syndrome is characterized by an increased risk of certain medical conditions, including heart defects, respiratory problems, and an increased risk of leukemia due to the abnormal chromosomal material. The condition also affects the immune system, with an increased risk of infections and autoimmune disorders.

**Why Each Wrong Option is Incorrect**
**Option A:** Heart defects are a common complication of Down's syndrome, with approximately 50% of individuals affected. Therefore, heart defects are indeed associated with Down's syndrome.
**Option B:** Respiratory problems, such as sleep apnea and chronic obstructive pulmonary disease (COPD), are also common in individuals with Down's syndrome. Thus, respiratory problems are associated with Down's syndrome.
**Option E:** Leukemia, specifically acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML), is a known risk factor in individuals with Down's syndrome. Therefore, leukemia is associated with Down's syndrome.

**Clinical Pearl / High-Yield Fact**
Individuals with Down's syndrome have an increased risk of developing Alzheimer's disease, with many showing signs of the condition by age 40. This is due to the presence of the amyloid precursor protein (APP) gene on chromosome 21, which is associated with the development of Alzheimer's disease.

**Correct Answer:** D