Atresia (plural: atresias) refers to a situation where there is absence, underdevelopment or abnormal closure, of a normal anatomical tubular structure or opening.

Contrast this with agenesis which refers to the complete absence of any anatomical structure including its primordial precursors.

For example:

Some authors occasionally overlap the two terms (i.e. atresia and agenesis), which in general is an erroneous practice.

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