Which structure is not palpable during a rectal examination in a healthy dog?

During a rectal examination in a healthy dog, the pelvic brim is not palpable. The pelvic brim is the bony structure that forms the boundary between the abdominal cavity and the pelvic cavity. It is located above the level of the rectum and cannot be felt through rectal palpation, as it is not accessible through the tissues of the rectal wall.

In contrast, the prostate gland, colon, and uterus are all structures that can be palpated during a rectal examination. The prostate gland is located near the rectum and can be felt readily in male dogs. The colon, being a portion of the intestines, is also located within reach during this examination, allowing for assessment of its contents and potential abnormalities. In females, the uterus can also be palpated, particularly if there are any notable changes such as enlargement due to pregnancy or pathology.

Thus, the pelvic brim is the only structure listed that would not be detected during a rectal examination of a healthy dog.