**Core Concept**
Down's syndrome is a genetic disorder characterized by an extra copy of chromosome 21, leading to various physical and developmental abnormalities. These abnormalities result from the altered expression of genes on chromosome 21, affecting multiple systems in the body.

**Why the Correct Answer is Right**
The correct answer is D. All, as individuals with Down's syndrome often exhibit a combination of physical features, including a gap between the first and second toes (sandal gap), clinodactyly (curved or deviated fingers), and hypotonia (low muscle tone). The presence of these features is a result of the genetic changes associated with Down's syndrome. Clinodactyly, in particular, is a common finding in individuals with Down's syndrome, and is thought to be related to the altered expression of genes involved in limb development. The sandal gap, also known as a gap between the first and second toes, is another characteristic feature of Down's syndrome, which is thought to result from the altered development of the first and second toes. Hypotonia, or low muscle tone, is a common finding in individuals with Down's syndrome, and is thought to result from the altered expression of genes involved in muscle development and function.

**Why Each Wrong Option is Incorrect**
**Option A:** The gap between the first and second toes is indeed a characteristic feature of Down's syndrome, and is therefore not incorrect.
**Option B:** Clinodactyly is also a characteristic feature of Down's syndrome, and is therefore not incorrect.
**Option C:** Hypotonia is a common finding in individuals with Down's syndrome, and is therefore not incorrect.

**Clinical Pearl / High-Yield Fact**
Individuals with Down's syndrome are at increased risk of developing various medical conditions, including heart defects, respiratory problems, and Alzheimer's disease. It is essential for healthcare providers to be aware of these risks and to provide regular screenings and monitoring to ensure optimal health outcomes.

**✓ Correct Answer: D. All**