**Core Concept**
Mutations in DNA are the primary cause of genetic disorders and can be triggered by various factors, including environmental exposures and errors during DNA replication. These changes can occur in the nucleotide sequence of DNA, leading to alterations in protein synthesis and function.

**Why the Correct Answer is Right**
The correct answer is related to the concept of nucleotide sequence changes. Mutations can occur due to errors during DNA replication, repair, or as a result of environmental exposures such as ultraviolet (UV) radiation, which can cause thymine dimers. These changes can lead to point mutations, insertions, deletions, or frameshift mutations, which can disrupt gene function and lead to disease.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because mutations can occur in the nucleotide sequence of DNA, not just in the expression of genes.
**Option B:** This option is incorrect because while epigenetic changes can affect gene expression, they do not directly cause mutations in the DNA sequence.
**Option C:** This option is incorrect because gene duplication is a type of genetic variation, but it is not a direct cause of mutations.

**Clinical Pearl / High-Yield Fact**
One important concept to remember is that mutations can occur due to errors during DNA replication, which can be triggered by factors such as UV radiation, oxidative stress, and errors in DNA repair mechanisms.

**Correct Answer:** D. DNA nucleotide sequence