Urachal cyst
（重定向自Cyst of urachus）
A urachal cyst is a sinus remaining from the allantois during embryogenesis. It is a cyst which occurs in the remnants between the umbilicus and bladder. This is a type of cyst occurring in a persistent portion of the urachus, presenting as an extraperitoneal mass in the umbilical region. It is characterized by abdominal pain, and fever if infected. It may rupture, leading to peritonitis, or it may drain through the umbilicus. Urachal cysts are usually silent clinically until infection, calculi or adenocarcinoma develop.