A newborn with Down syndrome has constipation. Rectal examination shows no stool in the rectal ampulla. The proximal rectosigmoid is dilated on radiograph. The mechanism most likely responsible for the patient’s disease is
**Core Concept**
The underlying condition is likely Hirschsprung's disease, a congenital disorder affecting the colon's ability to function due to the absence of ganglion cells in the distal colon and rectum. This leads to a functional obstruction, causing symptoms such as constipation and dilated proximal colon.
**Why the Correct Answer is Right**
In Hirschsprung's disease, the absence of ganglion cells in the rectosigmoid region results in a failure of the normal peristaltic movement of the colon. This leads to a functional obstruction, causing the proximal colon to dilate. The absence of stool in the rectal ampulla is a classic finding in this condition, as the affected segment of the colon is incapable of propelling stool into the rectum. The clinical presentation in a newborn with Down syndrome, who is at increased risk of Hirschsprung's disease, further supports this diagnosis.
**Why Each Wrong Option is Incorrect**
**Option A:** This option does not provide a plausible explanation for the patient's symptoms and findings. While it is a type of gastrointestinal obstruction, it is not directly related to the absence of ganglion cells in the colon.
**Option B:** This option refers to a different type of gastrointestinal obstruction caused by a mechanical blockage, such as intussusception or volvulus. However, the patient's symptoms and radiographic findings are more consistent with a functional obstruction, rather than a mechanical one.
**Option C:** This option is not directly related to the patient's symptoms and findings. While it is a type of gastrointestinal motility disorder, it is not typically associated with the absence of ganglion cells in the colon.
**Option D:** This option is not a valid diagnosis for the patient's symptoms and findings. While it is a type of gastrointestinal condition, it is not directly related to the absence of ganglion cells in the colon.
**Clinical Pearl / High-Yield Fact**
Hirschsprung's disease is a congenital condition that can be associated with Down syndrome, and it is essential to consider this diagnosis in newborns with constipation and dilated proximal colon on radiograph.
**Correct Answer:** C.