WBR0844

Author	[[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Embryology
Sub Category	SubCategory::Musculoskeletal/Rheumatology
Prompt	[[Prompt::A 52 year old female patient 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	AnswerA::1st pouch
Answer A Explanation	AnswerAExp::The middle ear cavity, the eustachian tube, and the mastoid air cells are derived from the 1st branchial pouch.
Answer B	AnswerB::2nd pouch
Answer B Explanation	AnswerBExp::The epithelial lining of the palatine tonsils rare derived from the 2nd branchial pouch.
Answer C	AnswerC::Ventral wing of 3rd pouch
Answer C Explanation	AnswerCExp::The thymus is derived from the ventral wing of the 3rd branchial pouch.
Answer D	AnswerD::Dorsal wing of 3rd pouch
Answer D Explanation	AnswerDExp::The inferior parathyroid glands are derived from the dorsal wing of the 3rd branchial pouch.
Answer E	AnswerE::4th pouch
Answer E Explanation	AnswerEExp::The superior parathyroid glands are derived from the 4th branchial pouch.
Right Answer	RightAnswer::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. Educational Objective: References: ]]
Approved	Approved::No
Keyword	WBRKeyword::branchial, WBRKeyword::pouch, WBRKeyword::derivative, WBRKeyword::4th, WBRKeyword::ventral, WBRKeyword::wing, WBRKeyword::thymus, WBRKeyword::thymoma, WBRKeyword::myasthenia, WBRKeyword::gravis, WBRKeyword::autoimmune, WBRKeyword::antibodies, WBRKeyword::acetylcholine, WBRKeyword::receptors, WBRKeyword::receptor, WBRKeyword::chest, WBRKeyword::xray
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Order in Linked Questions	LinkedOrder::