**Core Concept**
Mifepristone is a synthetic steroid with potent antiglucocorticoid and antiprogestin activities. Its mechanism of action involves binding to the glucocorticoid receptor and progesterone receptor, which can lead to various side effects.

**Why the Correct Answer is Right**
Mifepristone's most common side effect is related to its antiglucocorticoid activity. It can cause adrenal insufficiency by blocking the glucocorticoid receptor, leading to an increase in adrenocorticotropic hormone (ACTH) levels. This results in increased production of cortisol precursors, causing side effects such as **hypokalemia**, **fatigue**, and **nausea**. Mifepristone also has antiprogestin effects, which can lead to side effects like **dysmenorrhea** and **menorrhagia**.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while nausea and vomiting are common side effects of mifepristone, they are not the most common side effect.
**Option B:** This option is incorrect because while dysmenorrhea and menorrhagia are related to mifepristone's antiprogestin effects, they are not the most common side effects.
**Option C:** This option is incorrect because while hypokalemia is a potential side effect of mifepristone due to its antiglucocorticoid activity, it is not the most common side effect.

**Clinical Pearl / High-Yield Fact**
Mifepristone's antiglucocorticoid activity can lead to adrenal insufficiency, which requires careful monitoring in patients undergoing treatment.

**Correct Answer:** D.