What is a body part that starts with the letter X?

A body part that starts with the letter X is the xiphoid process. The xiphoid process is a part of the sternum, namely the breastbone.

Another name for the xiphoid process is the xiphosternum. The xiphoid process begins as cartilage in an infant, and as a child develops into an adult, it slowly turns into bone. By a person's middle age, the xiphoid process fuses itself to the main sternum body. At this point, it also fuses itself to the shoulder blades, which are also known as the clavicles. A displaced xiphoid process can contribute to xiphodynia, which is characterized by upper chest pain.