WBR0320

Author	[[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Biochemistry
Sub Category	SubCategory::Neurology
Prompt	[[Prompt::A one year old boy is brought by his parents to the physician's office for failure to thrive. Upon further questioning, the mother explains that the child is aggressive, has abnormal movements, and is always chewing off his fingertips and lips. Physical examination is remarkable for swelling of the joints, spasticity, and hyperreflexia. Based on the illustration below, which enzyme is most likely deficient in this patient? ]]
Answer A	AnswerA::A
Answer A Explanation	AnswerAExp::"A" does not correspond to hypoxanthine-guanosine-phosphoribosyltransferase (HGPRT).
Answer B	AnswerB::B
Answer B Explanation	AnswerBExp::"B" corresponds to HGPRT.
Answer C	AnswerC::C
Answer C Explanation	AnswerCExp::"C" does not correspond to HGPRT.
Answer D	AnswerD::D
Answer D Explanation	AnswerDExp::"D" does not correspond to HGPRT. It corresponds to adenosine deaminase (ADA). Its absence causes severe combined immunodeficiency (SCID).
Answer E	AnswerE::E
Answer E Explanation	AnswerEExp::"E" does not correspond to HGPRT.
Right Answer	RightAnswer::B
Explanation	[[Explanation::The patient has symptoms and signs consistent with Lesch-Nyhan syndrome, an X-linked disorder characterized by the absence of hypoxanthine-ribosyl-phosphoribosyltransferase (HGPRT), which normally converts hypoxanthine to inosinic acid (IMP) and guanine to guanylic acid (GMP). As a result, the purine salve pathway is lost and the patient has to undergo de novo purine synthesis. Excessive uric acid production in urea and increase in purine synthesis enzymes, along with the absence of any HGPRT activity and hyperuricemia, are present. Patients typically present with mental retardation, failure to thrive, choreoathetosis, and self-aggressive behavior with self-mutilation. Joint swelling is due to gout due to hyperuricemia. Educational Objective: Absence of HGPRT causes Lesch-Nyhan syndrome, characterized by mental retardation, self-mutilation, hyperuricemia, and choreoathetosis. Educational Objective: References: ]]
Approved	Approved::No
Keyword	WBRKeyword::HGPRT, WBRKeyword::lesch, WBRKeyword::nyhan, WBRKeyword::syndrome, WBRKeyword::self-mutilation, WBRKeyword::self, WBRKeyword::mutilation, WBRKeyword::aggressive, WBRKeyword::behavior, WBRKeyword::aggression, WBRKeyword::hyperuricemia, WBRKeyword::gout, WBRKeyword::hypoxanthine, WBRKeyword::guanine, WBRKeyword::phosphoribosyltransferase, WBRKeyword::purine, WBRKeyword::salvage, WBRKeyword::de, WBRKeyword::novo, WBRKeyword::X-linked, WBRKeyword::congenital, WBRKeyword::disorder
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::