WBR0844

Author	[[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Serge Korjian)]]
Exam Type	USMLE Step 1
Main Category	Embryology
Sub Category	Musculoskeletal/Rheumatology
Prompt	[[Prompt::A 52-year-old woman presents to the physician's office for muscle weakness and ptosis. Following initial work-up, the physician explains that her condition is caused by an autoimmune process whereby antibodies target post-synaptic acetylcholine receptors. Additional investigation reveals an abnormal structure marked by the red arrow in the chest X-ray shown below. The structure is most likely an embryological derivative of which branchial pouch? ]]
Answer A	1st pouch
Answer A Explanation	The middle ear cavity, the eustachian tube, and the mastoid air cells are derived from the 1st branchial pouch.
Answer B	2nd pouch
Answer B Explanation	The epithelial lining of the palatine tonsils rare derived from the 2nd branchial pouch.
Answer C	Ventral wing of 3rd pouch
Answer C Explanation	The thymus is derived from the ventral wing of the 3rd branchial pouch.
Answer D	Dorsal wing of 3rd pouch
Answer D Explanation	The inferior parathyroid glands are derived from the dorsal wing of the 3rd branchial pouch.
Answer E	4th pouch
Answer E Explanation	The superior parathyroid glands are derived from the 4th branchial pouch.
Right Answer	C
Explanation	[[Explanation::The patient is most likely diagnosed with myasthenia gravis, an autoimmune disease characterized by the presence of auto-antibodies against post-synaptic acetylcholine receptors. The disease is strongly associated with the presence of thymoma, a thymic tumor of epithelial origin. As such, a chest X-ray is always indicated whenever the diagnosis of myasthenia gravis is made. In the X-ray shown above, a thymoma is marked by the red arrow. The thymus is an encapsulated lobulated endodermal gland that is the site for T-cell differentiation and maturation. It is derived from the ventral wing of the 3rd branchial pouch. Educational Objective: The thymus is derived from the ventral wing of the 3rd branchial pouch. Thymoma is a tumor originating from the epithelial cells of the thymus that is associated with myasthenia gravis. References: Image courtesy of Dr Frank Gaillard. Radiopaedia (original file here). Creative Commons BY-SA-NC Dudek RW. High-Yield Embryology. Lww; 2013.]]
Approved	Yes
Keyword	Branchial pouch, Pouches, Embrology, Thymus, Thymoma, Myasthenia gravis, Autoimmune, Antibodies, Acetylcholine receptors
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