USMLE (United States Medical Licensing Examination): Step 1: Basic sciences assessment Practice Exam 35
Practice exam for USMLE (United States Medical Licensing Examination): Step 1: Basic sciences assessment.
Question 1
MEDIUMA 2-day-old male newborn is evaluated for progressive abdominal distension and bilious vomiting since birth. Pregnancy was uncomplicated. He has not passed meconium. Physical examination shows a distended abdomen with visible peristaltic waves. Abdominal radiograph shows multiple dilated bowel loops with air-fluid levels and no air in the rectum. A rectal suction biopsy shows absence of ganglion cells in the submucosal (Meissner) plexus. Which of the following embryologic events most likely failed to occur in this patient?
This presentation is consistent with Hirschsprung disease (aganglionic megacolon), due to failure of neural crest cell migration into the distal colon, leading to absent enteric ganglia and functional obstruction. Persistence of the vitelline duct causes Meckel diverticulum; failure of midgut return causes omphalocele; incomplete urorectal septum fusion causes anorectal malformations; cranial neural tube closure failure causes anencephaly.
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