All Sample Questions

Anal Canal 1

Anal canal


Question: A 1-year-old boy presents with a persistent anal fistula. The mother reports that her son has been having recurrent bouts of diarrhea and anal itching. He was born at term without any complications. On examination, a small opening is seen near the anus. The physician suspects a congenital disorder. Which embryologic structure failed to obliterate leading to the boy's condition?


A) Allantois

B) Ductus venosus

C) Ductus arteriosus

D) Foramen ovale

E) Umbilical arteries


A) Allantois


The allantois is an embryologic structure that plays an essential role in the formation of the urinary and gastrointestinal systems. It contributes to the formation of the urinary bladder and the urachus, a canal that drains the urinary bladder of the fetus into the allantois. Normally, after birth, the urachus obliterates and becomes the median umbilical ligament. However, if it fails to obliterate, several anomalies can occur, including a urachal cyst, patent urachus, or a urachal sinus, which can extend to the umbilicus or in some cases toward the rectum and present as a perianal fistula, which the boy in the question likely has. The other choices listed - the ductus venosus, ductus arteriosus, foramen ovale, and umbilical arteries - are all structures associated with fetal circulation, not the urinary or gastrointestinal systems.


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