**Core Concept**
Single gene disorders, also known as monogenic disorders, are caused by mutations in a single gene and can be inherited in an autosomal dominant, autosomal recessive, or X-linked pattern. These disorders often result from mutations that disrupt the function of the encoded protein or lead to the production of a non-functional protein. Examples of single gene disorders include sickle cell anemia, cystic fibrosis, and Huntington's disease.

**Why the Correct Answer is Right**
Single gene disorders are typically caused by mutations in a single gene, which can be identified through genetic testing. The correct answer is a disorder that is known to be caused by multiple genetic or environmental factors, rather than a single gene mutation.

**Why Each Wrong Option is Incorrect**
**Option A:** This is a single gene disorder caused by a mutation in the HBB gene, which encodes the beta-globin subunit of hemoglobin. It is inherited in an autosomal recessive pattern and is a classic example of a single gene disorder.

**Option B:** This is a single gene disorder caused by a mutation in the CFTR gene, which encodes the cystic fibrosis transmembrane conductance regulator protein. It is inherited in an autosomal recessive pattern and is a well-known example of a single gene disorder.

**Option C:** This is a single gene disorder caused by a mutation in the HTT gene, which encodes the huntingtin protein. It is inherited in an autosomal dominant pattern and is a classic example of a single gene disorder.

**Option D:** This is a complex disorder that is caused by multiple genetic and environmental factors, including genetic predisposition, lifestyle factors, and exposure to certain toxins. It is not caused by a single gene mutation and is therefore not a single gene disorder.

**Clinical Pearl / High-Yield Fact**
Be aware that many complex disorders have a strong genetic component and can be influenced by multiple genetic and environmental factors. Always consider the interplay between genetic and environmental factors when evaluating a patient's risk for a particular disorder.

**Correct Answer: D. This is a complex disorder that is caused by multiple genetic and environmental factors, including genetic predisposition, lifestyle factors, and exposure to certain toxins.**