What medical term describes the closure or absence of a normal body opening?

Atresia is the correct term that refers specifically to the closure or absence of a normal body opening. It is often used in medical contexts to describe conditions where there is either a congenital defect or an acquired obstruction preventing an opening from forming or remaining open. For example, esophageal atresia is a condition where the esophagus does not develop properly and ends in a blind pouch instead of connecting to the stomach, whereas anal atresia involves the absence of a normal anal opening.

The other terms provided do not relate to the concept of a closure or absence of a body opening. Fetal alcohol syndrome refers to a group of physical and mental disorders in a person caused by alcohol exposure while in the womb. Geriatrics is the branch of medicine dealing with the health care of elderly people, focusing on the process of aging and associated health issues. Idiopathic is a term used to describe conditions or diseases that arise spontaneously or for which the cause is unknown, rather than referring to a physical state of being such as atresia.