Solution: Acute appendicitis. US is the first line modality for evaluating children with symptoms suggestive of appendicitis. If the appendix is identified, and is non-compressible, measuring greater than 6 mm in diameter, the specificity is only 64%. If the appendix is greater than 7 mm, then the specificity increases to 88%. Hypervascular flow within the wall of the abnormal appendix increases the specificity to greater than 90%. However, a normal appendix is often difficult to identify at US and CT is often used in children where the appendix is not visualized, as well as in adults. CT is both highly sensitive and specific for appendicitis. In fact, if the appendix is not identified at CT, but there is not abnormal inflammatory process identified in the RLQ, this has been shown to have a very high negative predictive value for appendicitis.