Final answer:
The uterosacral ligament holds the uterus in place at the base of the spine and can cause lower back pain. The sacroiliac joint also relates to the region, but it supports sacrum and ilium connections.
Step-by-step explanation:
The structure that holds the uterus in place at the base of the spine and may be a source of low back pain is the uterosacral ligament. This ligament stabilizes the uterus posteriorly by connecting the cervix to the sacrum and pelvic wall. It keeps the uterus aligned within the abdominopelvic cavity, along with other supportive ligaments such as the broad ligament and the round ligament. The sacroiliac joint is supported by the anterior and posterior sacroiliac ligaments and while it also connects to the sacrum, it primarily assists in connecting the ilium portions of the hip bone and is less likely to cause uterine-related back pain.