WBR0980

Author	[[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Biochemistry
Sub Category	SubCategory::Renal
Prompt	[[Prompt::A 10 year old girl is brought by her mother to the emergency department for severe right flank pain radiating to her groin. The pain started suddenly a couple of hours ago and quickly became very severe. The mother reports that her daughter vomited once today. Urinalysis reveals few red blood cells in the urine in addition to hexagonal crystals. The level of which of the following is probably elevated in the urine of this girl?]]
Answer A	AnswerA::Lysine
Answer A Explanation	[[AnswerAExp::The symptoms and urinalysis findings of this patient are consistent with cystine kidney stones in the context of cystinuria. Lysine levels are probably elevated in the urine due to the impaired reabsorption of this amino acid by the mutated amino acid transporter.]]
Answer B	AnswerB::Phenylalanine
Answer B Explanation	[[AnswerBExp::The symptoms and urinalysis findings of this patient are consistent with cystine kidney stones in the context of cystinuria. Phenylalanine reabsorption is not impaired in this condition; therefore, the urine level of phenylalanine is not likely elevated.]]
Answer C	AnswerC::Tryptophan
Answer C Explanation	[[AnswerCExp::The symptoms and urinalysis findings of this patient are consistent with cystine kidney stones in the context of cystinuria. Tryptophan reabsorption is not impaired in this condition; therefore, the urine level of tryptophan is not likely elevated. Accumulation of tryptophan occurs in hartnup disease.]]
Answer D	AnswerD::Homocysteine
Answer D Explanation	[[AnswerDExp::The symptoms and urinalysis findings of this patient are consistent with cystine kidney stones in the context of cystinuria. Homocysteine reabsorption is not impaired in this condition; therefore, the urine level of homocysteine is not likely elevated. Homocysteine level is elevated in homocysteinuria.]]
Answer E	AnswerE::Leucine
Answer E Explanation	[[AnswerEExp::The symptoms and urinalysis findings of this patient are consistent with cystine kidney stones in the context of cystinuria. Leucine reabsorption is not impaired in this condition; therefore, the urine level of leucine is not likely elevated. Leucine level is elevated in maple syrup disease.]]
Right Answer	RightAnswer::A
Explanation	[[Explanation::The patient presents with symptoms of kidney stones. The urinalysis reveals hexagonal crystal consistent with cystine stone which occurs in patients with cystinuria. Cystinuria is autosomal recessive disease characterized by abnormal transporters in the kidneys for cysteine and other positively charged amino acids such as ornithine, lysine and arginine leading to impaired absorption of these amino acids. Patients with cystinuria are predisposed to cystine stones (cystine is formed of two cysteine linked by disulfide bond). The treatment of cystine stones is good hydration and urine alkalinization. Educational objective: Cystinuria is an autosomal recessive disease characterized by abnormal reabsorption of cysteine, ornithine, lysine and ornithine (mnemonic: cola) in the proximal convoluted tubules of the kidneys. Reference: First aid for USMLE step 1, 2013. Page 109. Educational Objective: References: ]]
Approved	Approved::No
Keyword	WBRKeyword::Cystinuria, WBRKeyword::cysteine
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::