Median Sacral Crest
The structure indicated is the median sacral crest.
The median sacral crest is located on the dorsal surface of the sacrum. The median sacral crest possesses three to four tubercles which are essentially the equivalent of the spinous processes of the upper sacral vertebrae.
Either side of the median sacral crest on the dorsal surface of the sacrum, are the posterior sacral foramina (four pairs). Inferiorly, the sacrum forms two horns, or cornua, which articulate with the cornua of the coccyx.
To learn more about the anatomy of the bones of the pelvis watch this video.