A 23-year-old man comes to the office with a 2-week history of malaise, loss of appetite, and vague abdominal pain. The patient has had no nausea, vomiting, changes in his bowel habits, dysuria, urinary frequency, or hematuria. He had an appendectomy for acute appendicitis at age 15; his medical history is otherwise unremarkable. He does not use tobacco or alcohol and has had no recent travel. His father was diagnosed with colon cancer at age 60. Physical examination reveals normal bowel sounds and no guarding or rebound tenderness. Abdominal CT scan obtained as part of this patient's evaluation is shown in the image below. The anatomical structure indicated by the arrow originates from which of the following embryologic divisions?

Question

A 23-year-old man comes to the office with a 2-week history of malaise, loss of appetite, and vague abdominal pain.  The patient has had no nausea, vomiting, changes in his bowel habits, dysuria, urinary frequency, or hematuria.  He had an appendectomy for acute appendicitis at age 15; his medical history is otherwise unremarkable.  He does not use tobacco or alcohol and has had no recent travel.  His father was diagnosed with colon cancer at age 60.  Physical examination reveals normal bowel sounds and no guarding or rebound tenderness.  Abdominal CT scan obtained as part of this patient's evaluation is shown in the image below.  The anatomical structure indicated by the arrow originates from which of the following embryologic divisions?

Quizplus · Accepted Answer

11ec160b_3741_72b2_9249_5da8a4f1b568_MD0001_00 The arrow in this patient's abdominal CT scan is pointing to the spleen, a large, wedge-shaped lymphatic organ that is situated in the posterior superior portion of the left abdominal cavity. While most gut tissue is endodermal in origin, the spleen is unique as it is derived from condensed mesenchymal tissue in the dorsal mesentery during embryonic development. Mesenchymal tissue arises from the mesoderm germ layer and is characterized by loosely associated cells surrounded by the extracellular matrix. (Choice E) The notochord is a mesodermally derived structure that almost completely regresses in humans, and its only major derivative is the nucleus pulposus of the intervertebral disc. Other major derivatives of the mesoderm germ layer include muscle, bone, lymphatics, the cardiovascular system, and kidneys/ureters. Educational objective: The spleen is a large, wedge-shaped lymphatic organ that is situated in the posterior superior portion of the left abdominal cavity. It is derived from mesoderm in the dorsal mesentery during embryonic development. __________ References: Embryonic origins of spleen asymmetry. (http://www.ncbi.nlm.nih.gov/pubmed/10654610)

A 23-Year-Old Man Comes to the Office with a 2-Week