WBR0320

Author	[[PageAuthor::Rim Halaby, M.D. [1], Alison Leibowitz [2] (Reviewed by Alison Leibowitz)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Biochemistry
Sub Category	SubCategory::Neurology
Prompt	[[Prompt::Brought by his parents, a 1-year-old male presents to the physician's office for failure to thrive. Upon further questioning, the mother explains that the child is aggressive, moves abnormally, and frequently chews his fingertips and lips. Upon physical examination, you observe a swelling of the first metatarsophalangeal joint, spasticity, and hyperreflexia. Which of the following enzymes in the diagram is most likely deficient in this patient? ]]
Answer A	AnswerA::A
Answer A Explanation	AnswerAExp::"A" does not correspond to 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 demonstrates symptoms and signs indicative of Lesch-Nyhan syndrome, an X-linked disorder characterized by the absence of hypoxanthine-ribosyl-phosphoribosyltransferase (HGPRT), which converts hypoxanthine to inosine monophosphate (IMP), guanine to guanosine monophasphate (GMP), and xanthine to xanthosine monophosphate. As a result, the purine salvage pathway is hindered and the patient has to undergo de novo purine synthesis. Lesch-Nyhan syndrome manifests with excessive uric acid production in urea and an increase in purine synthesis, leading to to hyperuricemia. Patients typically present with mental retardation, failure to thrive, choreoathetosis, and self-mutilation. Joint swelling results from gout, due to hyperuricemia. Educational Objective: Absence of HGPRT leads to Lesch-Nyhan syndrome, characterized by mental retardation, self-mutilation, hyperuricemia, and choreoathetosis. References: ]]
Approved	Approved::Yes
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::