WBR0138

Author	[[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Embryology
Sub Category	SubCategory::Neurology
Prompt	[[Prompt::A 23-year-old male was referred to a neurologist for the evaluation of progressive worsening vertigo and hearing loss that have started almost 4 months ago. Brain MRI showed bilateral cerebello-pontine angle masses. What is the embryological origin of the cells from which this patient’s masses arised?]]
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 part of the mouth, pharynx and the terminal part of the rectum (which are lined by involutions of the ectoderm), 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) along with glands, hair, and nails Epithelium of the mouth and nasal cavity salivary glands, and glands of mouth and nasal cavity Tooth enamel (as a side note, dentin and dental pulp are derived from ectoderm) Epithelium of anterior pituitary Lens, cornea, lacrimal gland, tarsal glands and the conjunctiva of the eye Apical ectodermal ridge inducing development of the limb buds of the 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 developing heart Ciliary body of the eye Adrenal medulla]]
Right Answer	RightAnswer::E
Explanation	[[Explanation::The patient presents with bilateral masses compressing CN VIII and causing hearing symptoms and vertigo. The bilateral nature of the masses, the age of the patient and the involvement of the CN VIII suggest a diagnosis 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 embryological origin of shwannomas is the neural crest. Educational Objective: References: ]]
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