WBR232

Author	[[PageAuthor::Rim Halaby, M.D. [1] (Reviewed by Will Gibson)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Pathology
Sub Category	SubCategory::Renal
Prompt	[[Prompt::A 24 year old Caucasian male patient presents to the emergency department with back pain that radiates to the inguinal region. The pain started on the day of presentation. On further questioning, the patient reveals he has had recurrent kidney stones since the age of 11. The patient’s urine smells like rotten eggs. Appropriate work-up done prior to presentation has shown elevated levels of urinary arginine, ornithine, and lysine. Which of the following treatment modalities is the best option for this patient’s renal colic?]]
Answer A	AnswerA::Parathyroidectomy
Answer A Explanation	[[AnswerAExp::Parathyroidectomy would be an appropriate treatment option for patients with primary hyperparathyroidism who have recurrent calcium-containing stones.]]
Answer B	AnswerB::Carbonic anhydrase inhibitor
Answer B Explanation	AnswerBExp::Carbonic anhydrase inhibitors are used in the management of cystine stones.
Answer C	AnswerC::Protein synthesis inhibitor that acts on bacterial 30S ribosomal subunit
Answer C Explanation	[[AnswerCExp::Antimicrobials are not helpful in cystine stones. Antimicrobials are helpful in urinary tract infections that might in turn predispose to magnesium ammonium phosphate (MAG) struvite stones. These bacteria precipitate struvite stones by splitting urea into ammonium.]]
Answer D	AnswerD::Xanthine oxidase inhibitor
Answer D Explanation	[[AnswerDExp::Xanthine oxidase inhibitors, such as allopurinol, are helpful in the treatment of kidney stones with a uric acid component.]]
Answer E	AnswerE::Na-K-2Cl co-transport inhibitor at the level of loop of Henle
Answer E Explanation	[[AnswerEExp::Loop diuretics are Na-K-2Cl inhibitors; they have no role in the treatment of cystine stones.]]
Right Answer	RightAnswer::B
Explanation	[[Explanation::Cystinuria is an autosomal recessive inherited disorder characterized by defect in intestinal absorption and renal reabsorption of cystine and dibasic amino acids across the luminal membrane of the renal proximal tubule. Elevated levels of urinary cystine will subsequently crystallize and form stones. The clinical symptoms of cystine stones are identical to other kidney stones (ie severe flank pain). Typically, patients with cystinuria complain of recurrent kidney stones starting the first 2 decades of life and often have a positive family history of stone disease. Urinalysis shows pathognomonic hexagonal crystals that contain sulfur amino acids due to the presence of cystine. Characteristic rotten egg urine smell is also attributed to the sulfur-containing compounds in cystine stones. Initial management of cystine stones includes increased fluid intake, urinary alkalinization with acetazolamide, a carbonic anhydrase inhibitor, and thiol medications to reduce the urinary cystine concentration. However, once stones have formed, the kinetics of crystal dissolution are unfavorable. Extracorporeal shock wave lithotripsy or surgery are options for patients with symptomatic stones. Educational Objective: Cystinuria is an autosomal recessive disorder characterized by impaired GI absorption and renal reabsorption of cystine and dibasic amino acids. Cystine stones contain sulfur containing compounds that give a characteristic rotten egg smell. Treatment includes alkalinization of urine by acetazolamide, a carbonic anhydrase inhibitor. References: First Aid 2015 page 108]]
Approved	Approved::Yes
Keyword	WBRKeyword::Biochemistry, WBRKeyword::Genetics, WBRKeyword::Metabolism, WBRKeyword::Amino acids, WBRKeyword::Amino acid metabolism, WBRKeyword::Kidney stones, WBRKeyword::Nephrolithiasis, WBRKeyword::Cystine, WBRKeyword::Cystinuria
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