Ala of sacrum is not crossed by
**Core Concept**
The ala of the sacrum is a wing-like structure that forms the posterior boundary of the pelvic cavity. Understanding the anatomy of the sacrum, including its various parts and their relationships with surrounding structures, is crucial for diagnosing and managing various pelvic and lower limb disorders.
**Why the Correct Answer is Right**
The ala of the sacrum is formed by the lateral parts of the sacral alar wings, which are made up of the sacral wings and the iliac crests. The sacral nerves, which arise from the sacral spinal cord, exit the sacrum through the sacral foramina and are not typically associated with the ala of the sacrum. The sacral promontory, on the other hand, is a distinct landmark that serves as the point of attachment for the rectum and the sigmoid colon.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely incorrect because it does not specify a structure that is known to be associated with the ala of the sacrum.
**Option B:** This option is incorrect because the rectal ampulla, while closely associated with the sacrum, is not typically associated with the ala of the sacrum.
**Option C:** This option is likely incorrect because the iliac artery, while closely associated with the sacrum, is not typically associated with the ala of the sacrum.
**Clinical Pearl / High-Yield Fact**
The sacrum is a fused set of five sacral vertebrae that form the posterior boundary of the pelvic cavity. Understanding the anatomy of the sacrum, including its various parts and their relationships with surrounding structures, is crucial for diagnosing and managing various pelvic and lower limb disorders.
**Correct Answer:** D.