How many fused vertebrae make up the sacrum?

The sacrum is composed of five fused vertebrae in adults, designated as S1 to S5. These vertebrae are fused together to form a single, triangular bone at the base of the spine, connecting the spine to the pelvis. The fusion of these vertebrae occurs during adolescence and early adulthood, contributing to the stability and strength required for weight-bearing and movement in the pelvic region. This anatomical feature is significant as it provides the structural foundation for the human skeleton and facilitates the attachment for various muscles and ligaments in the lower back and pelvis.

In contrast, the options indicating three, four, or seven fused vertebrae do not reflect the typical fusion pattern seen in the human sacrum. The correct number of five is vital for understanding human anatomy, particularly in contexts such as physical health, injury assessment, and surgical procedures involving the lower spine.