Pectus excavatum is a congenital malformation caused by a deformity of the costal cartilages (fifth through eighth), which develop abnormally in a concave manner, pushing the sternum posteriorly. This chest deformity is most severe at the xiphoid.
Source: Blackbourne LH, Fleischer KJ, eds. Advanced Surgical Recall. Baltimore, Md: Williams & Wilkins; 1997:846.
October 2002. Arch Surg. 2002;137(10):1173. doi:10.1001/archsurg.137.10.1173