**Core Concept**
The fundus of the uterus is the uppermost portion, and its drainage is an essential aspect of the uterine anatomy and physiology. The fundus is connected to the uterine cavity via the uterine horns, and its drainage plays a crucial role in the menstrual cycle and reproductive health.

**Why the Correct Answer is Right**
The fundus of the uterus drains into the uterine horns, which then empty into the uterine cavity. This drainage is facilitated by the uterine contractions and the peristaltic movements of the uterine smooth muscle. The uterine horns are anatomically positioned to allow for efficient drainage of the uterine cavity, ensuring proper menstrual flow and reproductive function.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is a distractor that does not accurately describe the uterine anatomy or drainage pathway.
**Option B:** This option is incorrect because it is not a recognized anatomical structure related to the uterine drainage.
**Option C:** This option is incorrect because it is not the primary drainage pathway for the fundus of the uterus.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the uterine horns and drainage pathway play a critical role in the menstrual cycle and reproductive health. Any abnormalities in this pathway can lead to menstrual irregularities, infertility, or other reproductive issues.

**Correct Answer: C. Isthmus of the uterus.**