**Core Concept**
The cervical canal and the cavity of the body of the uterus are connected by the internal os, which is a narrow opening that allows for the passage of menstrual flow, fertilized eggs, and other substances. This connection is a critical aspect of female reproductive anatomy.

**Why the Correct Answer is Right**
The internal os is a circular opening in the cervix that connects the cervical canal to the uterine cavity. It is the primary route for menstrual flow and is also the site where a fertilized egg implants in the uterus. During pregnancy, the internal os remains closed, and during childbirth, it dilates to allow for the passage of the baby. The internal os is surrounded by the cervical stroma, which is rich in blood vessels and connective tissue.

**Why Each Wrong Option is Incorrect**
**Option A:** The external os is the opening of the cervix that leads to the vaginal canal, not the connection between the cervical canal and the uterine cavity. It is a smaller opening compared to the internal os.
**Option B:** The isthmus is a narrow region of the uterus that connects the isthmic portion of the uterine cavity to the cervix, but it is not the direct connection between the cervical canal and the uterine cavity.
**Option C:** The fundus is the upper portion of the uterus, but it is not the connection between the cervical canal and the uterine cavity.

**Clinical Pearl / High-Yield Fact**
The internal os is a critical structure in gynecology, and its patency can be assessed during a pelvic exam. A closed internal os is a sign of pregnancy, while an open internal os may indicate a miscarriage or an abnormal uterine cavity.

**Correct Answer:** A. Internal os.