WBR0138

Author	[[PageAuthor::Rim Halaby, M.D. [1] (Reviewed by Alison Leibowitz)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Embryology
Sub Category	SubCategory::Neurology
Prompt	[[Prompt::A 23-year-old male is referred to a neurologist for evaluation of a progressively worsening vertigo and hearing loss that started almost 4 months ago. A brain MRI showed bilateral cerebello-pontine angle masses. Which of the following is the most likely embryological origin of the cells from which this patient’s masses arose?]]
Answer A	AnswerA::Mesoderm
Answer A Explanation	[[AnswerAExp::The mesoderm gives rise to: Mesenchyme (connective tissue) Mesothelium Non-epithelial blood cells and coelomocytes Muscles Septa Mesenteries Part of the gonads (the rest being the gametes)]]
Answer B	AnswerB::Endoderm
Answer B Explanation	[[AnswerBExp::The endoderm gives rise to: GI: the entire alimentary canal except for part of the mouth, pharynx, the terminal part of the rectum (which are lined by involutions of the ectoderm), and the lining cells of all the glands which open into the digestive tube, including those of the liver and pancreas. Respiratory: the trachea, bronchi, and alveoli of the lungs Endocrine: the lining of the follicles of the thyroid gland and thymus Auditory system: epithelium of the auditory tube and tympanic cavity Urinary system: urinary bladder and part of the urethra]]
Answer C	AnswerC::Surface ectoderm
Answer C Explanation	[[AnswerCExp::The surface ectoderm gives rise to: Skin (only epidermis; dermis is derived from mesoderm), glands, hair, and nails Epithelium of the mouth and nasal cavity, salivary glands, and glands of mouth and nasal cavity Tooth enamel (although dentin and dental pulp are derived from ectoderm) Epithelium of the anterior pituitary Lens, cornea, lacrimal gland, tarsal glands, and the conjunctiva of the eye Apical ectodermal ridge-inducing development of the limb buds of an embryo Sensory receptors in epidermis]]
Answer D	AnswerD::Neuroectoderm
Answer D Explanation	[[AnswerDExp::The neuroectoderm gives rise to: Brain (rhombencephalon, mesencephalon, and prosencephalon) Spinal cord and motor neurons Retina Posterior pituitary]]
Answer E	AnswerE::Neural crest
Answer E Explanation	[[AnswerEExp::The neural crest gives rise to: Pigment cells in the skin Ganglia of the autonomic nervous system Dorsal root ganglia Facial cartilage Spiral septum of the developing heart Ciliary body of the eye Adrenal medulla]]
Right Answer	RightAnswer::E
Explanation	[[Explanation::The patient presents with bilateral masses, compressing CN VIII, which results in hearing loss and vertigo. The bilateral nature of the masses, the age of the patient, and the involvement of the CN VIII are all characteristic of neurofibromatosis type II. Hence, the masses are more likely bilateral vestibular shwannoma. The embryological origin of shwannomas is the neural crest. Educational Objective: The neural crest is the embryological origin of shwannomas. References: First Aid 2014 page 444]]
Approved	Approved::Yes
Keyword	WBRKeyword::Embryology, WBRKeyword::Neural crest, WBRKeyword::Schwannoma, WBRKeyword::Brain tumor, WBRKeyword::Cancer, WBRKeyword::Neurofibromatosis
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