Make the Diagnosis: Fall 2021

AB is an 11-year-old girl who presents to the emergency department, with her father, with a concern for constipation and abdominal pain. She states that she has had a history of constipation since birth and was prescribed MiraLAX® by her pediatrician in the past, although she had stopped taking it a few weeks ago. She is premenarchal. She describes her stools as intermittently hard, with some straining to evacuate; her last bowel movement just prior to coming to the ED was somewhat loose. She has had some lower abdominal pain recently, which is relieved by acetaminophen. She has not had any fevers or vomiting, and her appetite has otherwise been good.

On examination, AB is alert and well appearing. She is well developed and well nourished, and her vital signs are unremarkable. Her abdomen is soft with mild tenderness to palpation in the lower quadrants bilaterally and some suprapubic fullness; there is no rebound or guarding. Her neurologic examination is unremarkable, and there are no unusual skin rashes. On rectal examination, there is an empty rectal vault and a soft palpable mass anteriorly.

What is the most likely diagnosis?