WBR0103

Author	PageAuthor::William J Gibson
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Genetics
Sub Category	SubCategory::Endocrine, SubCategory::Musculoskeletal/Rheumatology
Prompt	[[Prompt::A 32 year old man with dwarfism has recently married a woman of normal stature. Neither of his parents had dwarfism, nor did any of his two siblings. He and his wife are interested in having children and seek genetic counseling? Which of the following statements is most likely true?]]
Answer A	AnswerA::The patient’s condition is caused by inactivating mutations in the FGFR3 gene and his children have a near 0% chance of developing dwarfism.
Answer A Explanation	[[AnswerAExp::Incorrect - While achondroplasia is caused by mutations of the FGFR3 gene, they are mutations which cause hyperactive signaling of this receptor. Achondroplasia is an autosomal dominant condition, thus 50% of his children will develop dwarfism.]]
Answer B	AnswerB::he patient’s condition is caused by activating mutations in the FGFR3 gene and his children have a 50% chance of developing dwarfism.
Answer B Explanation	AnswerBExp::Correct - See explanation
Answer C	AnswerC::The patient’s condition is caused by defects in Type I Collagen and his children have a 50% chance of developing dwarfism.
Answer C Explanation	AnswerCExp::Incorrect - While Type I collagen is responsible for constituting bone, defects in Type I collagen are responsible for Osteogenesis Imperfecta, not achondroplasia.
Answer D	AnswerD::The patient’s condition is caused by defects in Growth Hormone synthesis and his children have a 0% chance of developing dwarfism.
Answer D Explanation	AnswerDExp::Incorrect - While growth hormone deficiency can cause dwarfism, achondroplasia is more common.
Answer E	AnswerE::The patient’s condition is caused by defects in Growth Hormone receptor signaling and his children have a 50% chance of developing dwarfism.
Answer E Explanation	AnswerEExp::Incorrect - While defects in the growth hormone receptor can cause dwarfism, they are extremely rare.
Right Answer	RightAnswer::B
Explanation	[[Explanation::Achondroplasia is caused by defects in the FGFR3 gene which cause gain of function. The activated receptor continually signals for bone to cease growing. Educational Objective: References: ]]
Approved	Approved::Yes
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