Final answer:
The most common cause of congenital bilateral absence of the vas deferens is cystic fibrosis.
Step-by-step explanation:
The most common cause of congenital bilateral absence of the vas deferens (CBAVD) is a condition called cystic fibrosis, which is a genetic disorder. Cystic fibrosis affects the functioning of various organs, including the reproductive system. In individuals with CBAVD, the vas deferens, which is responsible for transporting sperm, is missing or blocked.
For example, mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene can lead to abnormal secretion of chloride ions and thick mucus production, which can block the vas deferens. This condition is more commonly observed in males with cystic fibrosis.
It is important to note that not all individuals with CBAVD have cystic fibrosis, but a significant proportion of them do. Other factors, such as genetic variations in other genes involved in the development of the reproductive system, can also contribute to the absence or blockage of the vas deferens in some cases.