**Core Concept**
Mild cognitive impairment (MCI) is a condition characterized by a decline in cognitive function beyond what is expected for an individual's age and education level, but not severe enough to interfere with daily life. The conversion rate from MCI to dementia refers to the proportion of individuals with MCI who eventually develop dementia over a specified period.

**Why the Correct Answer is Right**
Studies have consistently shown that the annual conversion rate to dementia in patients with MCI is around 10-15%. This is because MCI is considered a transitional phase between normal cognitive aging and dementia, with many individuals progressing to dementia over time. The risk of conversion is influenced by various factors, including age, education level, family history of dementia, and the presence of certain medical conditions such as hypertension, diabetes, and cardiovascular disease.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option does not provide a specific conversion rate, making it an unclear choice.
* **Option B:** There is no established annual conversion rate to dementia in patients with MCI of 20-30%.
* **Option C:** A conversion rate of 5% is too low, considering the established risk of progression from MCI to dementia.

**Clinical Pearl / High-Yield Fact**
It's essential to note that not all individuals with MCI will progress to dementia, and the risk of conversion can be reduced with interventions such as cognitive training, lifestyle modifications, and management of underlying medical conditions.

**Correct Answer:** C. 10-15%.