**Core Concept**
Down syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of an extra copy of chromosome 21. It is characterized by intellectual disability, delayed speech, and distinctive physical features. Individuals with Down syndrome often have a higher risk of developing various health complications.

**Why the Correct Answer is Right**
The correct answer is related to the common health issues associated with Down syndrome. Individuals with Down syndrome are more prone to infections, particularly those caused by encapsulated bacteria such as Streptococcus pneumoniae and Haemophilus influenzae. They also have a higher risk of developing autoimmune disorders, such as Hashimoto's thyroiditis, and leukemia. Furthermore, Down syndrome individuals are more susceptible to cardiovascular diseases, including atrioventricular septal defects and tetralogy of Fallot.

**Why Each Wrong Option is Incorrect**
* **Option A:** Individuals with Down syndrome have a higher risk of developing Alzheimer's disease, but this is not the correct answer since it is not the only option listed.
* **Option B:** Down syndrome individuals are more prone to infections, particularly those caused by encapsulated bacteria, but this is not the only option listed.
* **Option C:** Down syndrome individuals have a higher risk of developing leukemia, especially acute lymphoblastic leukemia (ALL), but this is not the only option listed.

**Clinical Pearl / High-Yield Fact**
Individuals with Down syndrome have a higher risk of developing thyroid abnormalities, with Hashimoto's thyroiditis being a common autoimmune thyroiditis. They also have a higher risk of developing celiac disease.

**Correct Answer:** D.