**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 extra genetic material disrupts normal development, leading to characteristic physical and cognitive features.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of Down's syndrome. Individuals with Down's syndrome have an increased risk of developing Alzheimer's disease, particularly in their 50s and 60s. This is due to the presence of an extra copy of the amyloid precursor protein (APP) gene on chromosome 21, which leads to the overproduction of amyloid beta peptides. These peptides accumulate in the brain, forming amyloid plaques and leading to neurodegeneration and cognitive decline.

**Why Each Wrong Option is Incorrect**
**Option A:** While individuals with Down's syndrome may have a higher risk of certain medical conditions, such as heart defects and leukemia, this is not the correct answer.
**Option B:** Down's syndrome is not directly related to an autoimmune disorder.
**Option C:** While individuals with Down's syndrome may have a higher risk of certain infections, this is not the correct answer.

**Clinical Pearl / High-Yield Fact**
Individuals with Down's syndrome have a higher risk of developing Alzheimer's disease due to the presence of an extra copy of the APP gene on chromosome 21. This highlights the importance of regular cognitive and neurological assessments in this population.

**Correct Answer: A. Increased risk of Alzheimer's disease.**