Where should the central ray be centered for the PA sacrum projection?

The central ray for the PA sacrum projection should indeed be centered on the S2 tubercle. This location is anatomically significant because the sacrum articulates at the sacroiliac joints, and the S2 level is located in the middle of the sacral region. Centering the ray at this point ensures that the entire sacrum is properly visualized while minimizing distortion and maximizing image clarity. It is important for achieving optimal imaging of the sacral structure, which is critical for diagnostic purposes.

While the other options are related to anatomy in the pelvic region, they do not align with the correct anatomical landmark for a PA sacrum projection. The ASIS (anterior superior iliac spine) is higher than the required position, L5 is too high and constitutes a different anatomical area, and centering 1" below the iliac crest may not accurately target the sacrum, potentially leading to a foreshortened or misaligned image of the sacral anatomy.