Vignette: A 6-month-old boy is being evaluated for a recurrent urinary tract infection. His mother reports that he has had three infections in the past two months. Physical examination reveals a palpable mass in the lower right abdomen. He is sent for an ultrasound, which shows a fluid-filled cystic mass in the lower right abdomen. Further imaging shows that the mass is connected to the bladder. Which embryological structure failed to involute, leading to this condition?
A. Ductus arteriosus
B. Ductus venosus
D. Left horn of sinus venosus
E. Septum primum
The urachus is an embryological structure that connects the urinary bladder of the fetus to the allantois, a structure involved in early fluid exchange between the embryo and the yolk sac. Normally, the urachus obliterates during fetal development and becomes a fibrous cord called the median umbilical ligament. If the urachus fails to obliterate, it may lead to a urachal cyst. In some cases, the cyst can become infected, leading to recurrent urinary tract infections. The mass in the lower right abdomen in this patient, and its connection to the bladder, suggest a urachal cyst. The other choices listed are embryological structures that normally involute or transform into adult structures, but their failure to do so would not result in the clinical picture presented.