A 27-year-old woman is admitted to the emergency department with markedly elevated temperature and abdominal pain. Physical examination initially indicates paraumbilical pain, but the site of origin of pain soon shifts to the right lower quadrant. A CT scan is shown. Which of the following structures is affected?

(b) Source: GAS 388; GA 158 Paraumbilical pain progressing into the right iliac fossa is a sign of appendicitis. The CT scan is one of an inflamed appendix. The structures in the scan that lie to the right of the vertebral body are part of the psoas muscle. Transverse section of this muscle signifies that it is a cut from the lumbar region.