What is the relevant anatomical landmark for locating the sacrum?

The iliac crests serve as a crucial anatomical landmark for locating the sacrum. Located at the top of the pelvis, the iliac crests are the superior borders of the ilium bones of the pelvis. When palpating the iliac crests, a practitioner can determine the level of the sacrum, as the sacral base typically aligns closely with where the iliac crests are palpated. This alignment assists healthcare professionals in assessing the sacrum's position and in performing techniques related to the sacrum.

In the context of the other options, while the navel can be used as a general reference point for the abdomen, it does not directly aid in locating the sacrum. The shoulder blades are positioned much higher in the body and are unrelated to pelvic alignment. Lastly, the lumbar vertebrae, while important for understanding the lower back, do not provide a direct anatomical marker for finding the sacrum. Thus, the iliac crests are the most relevant landmark for this purpose.