WBR0980

Author	[[PageAuthor::Rim Halaby, M.D. [1] (Reviewed by Yazan Daaboul, M.D.)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Biochemistry
Sub Category	SubCategory::Renal
Prompt	[[Prompt::A 10-year-old girl is brought to the emergency department (ED) for severe right flank pain that developed acutely over the past 2 hours. The patient explains that her pain radiates to her groin and is only associated with nausea and one episode of yellowish vomiting. When asked, she denies any association between her pain and food intake, change in bowel movement, or change in position. Physical examination is remarkable for costovertebral angle tenderness on the right side. Urinalysis in the ED is remarkable for numerous red blood cells in addition to the presence of hexagonal crystals. Chronic elevation in the urinary concentration of which compound is associated with the development of this patient's condition?]]
Answer A	AnswerA::Lysine
Answer A Explanation	[[AnswerAExp::The patient's presentation and findings on urinalysis are consistent with cystine stones, which often develop among patients with cystinuria. The urinary concentration of lysine is elevated due to the impaired renal reabsorption and excessive excretion of this amino acid by the mutated amino acid transporter.]]
Answer B	AnswerB::Phenylalanine
Answer B Explanation	[[AnswerBExp::The patient's presentation and findings on urinalysis are consistent with cystine stones, which often develop among patients with cystinuria. Phenylalanine reabsorption is not impaired in cystinuria, and urinary concentration of phenylalanine is not typically elevated.]]
Answer C	AnswerC::Tryptophan
Answer C Explanation	[[AnswerCExp::The patient's presentation and findings on urinalysis are consistent with cystine stones, which often develop among patients with cystinuria. Tryptophan reabsorption is not impaired in cystinuria, and urinary concentration of tryptophan is not typically elevated. Excessive urinary excretion and impaired intestinal absorption of tryptophan is characteristic of hartnup disease.]]
Answer D	AnswerD::Homocysteine
Answer D Explanation	[[AnswerDExp::The patient's presentation and findings on urinalysis are consistent with cystine stones, which often develop among patients with cystinuria. Homocysteine reabsorption is not impaired in cystinuria, and urinary concentration of homocysteine is not typically elevated. Serum and urine homocysteine concentrations are elevated in homocysteinuria.]]
Answer E	AnswerE::Leucine
Answer E Explanation	[[AnswerEExp::The patient's presentation and findings on urinalysis are consistent with cystine stones, which often develop among patients with cystinuria. Leucine reabsorption is not impaired in cystinuria, and urinary concentration of leucine is not typically elevated. Serum leucine, isoleucine, and valine (branched amino acids) concentrations are elevated in maple syrup urine disease (MSUD).]]
Right Answer	RightAnswer::A
Explanation	[[Explanation::The patient presents with signs and symptoms of kidney stones (nephrolithiasis). The urinalysis is remarkable for hexagonal crystals, which are consistent with the diagnosis of cystine kidney stones that typically develop among patients with cystinuria. Cystine is composed of two cysteine molecules linked by a disulfide bond. Cystinuria is an autosomal recessive genetic disease characterized by reduced renal reabsorption of positively charged amino acids. It is caused by the presence of abnormal transporters in the the proximal convoluted tubules for cysteine and other positively charged amino acids, such as ornithine, lysine, and arginine. Cystinuria results in impaired renal reabsorption and excessive excretion of these amino acids, and patients with cystinuria are predisposed to the development of cystine stones. Management of cystine stones is adequate hydration and urine alkalinization. Educational Objective: Cystinuria is an autosomal recessive genetic disease characterized by reduced reabsorption of positively charged amino acids. It is caused by the presence of abnormal transporters in the proximal convoluted tubules for cysteine and other positively charged amino acids such as ornithine, lysine and arginine. References: First Aid 2014 page 111]]
Approved	Approved::Yes
Keyword	WBRKeyword::Cystinuria, WBRKeyword::Cysteine, WBRKeyword::Cystine stone, WBRKeyword::Nephrolithiasis, WBRKeyword::Kidney stone, WBRKeyword::Transporter, WBRKeyword::Ornithine, WBRKeyword::Lysine, WBRKeyword::Arginine
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::