Atresia is a condition in which a body orifice or passage in the body is abnormally closed or completely absent. Atresia can affect many structures in the body, including:

• Anal atresia -- congenital absence of the anus (also called imperforate anus).
• Aortic atresia -- congenital absence of the valvular opening into the aorta.
• Aural Atresia -- the absence of an external ear canal, often with malformation of the external, middle or inner ear.
• Biliary atresia -- absence of the major bile ducts.
• Choanal atresia -- congenital failure of nasal passages to open.
• Duodenal atresia -- a condition in which the first part of the duodenum has not developed properly.
• Esophageal atresia -- a birth defect in which part of esophagus is not hollow.
• Intestinal atresia -- obliteration of the hollow of the small intestine, involving the ileum or the jejunum or duodenum.
• Laryngeal atresia -- congenital failure of the laryngeal opening to develop.
• Prepyloric atresia -- congenital membranous obstruction of the gastric outlet.
• Pulmonary atresia -- congenital absence of the pulmonary valve opening in the heart.
• Tricuspid atresia -- congenital lack of the tricuspid valve opening.
• Vaginal atresia -- congenital occlusion of the vagina or subsequence adhesion of the walls of the vagina occluding it.