WBR0618

Author	[[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Physiology
Sub Category	SubCategory::Renal
Prompt	[[Prompt::A researcher is studying the role of cimetidine, a H2 receptor blocker, in renal physiology. He conducts an experiment whereby he measures the glomerular filtration rate (GFR) in 20 volunteers by measuring creatinine clearance first then by measuring inulin clearance. Then, he gives the volunteers oral cimetidine and notes the change in GFR and clearances of both creatinine and inulin. Which of the following changes are most likely to occur following the oral intake of cimetidine? ]]
Answer A	AnswerA::A
Answer A Explanation	AnswerAExp::Creatinine secretion does not increase with use of cimetidine.
Answer B	AnswerB::B
Answer B Explanation	AnswerBExp::Creatinine secretion does not increase with use of cimetidine.
Answer C	AnswerC::C
Answer C Explanation	AnswerCExp::Cimetidine inhibits tubular secretion of creatinine. As such, creatinine clearance will decrease but calculated GFR based on creatinine will become closer to the true GFR, which is calculated by measuring inulin clearance.
Answer D	AnswerD::D
Answer D Explanation	AnswerDExp::Inulin clearance is not affected.
Answer E	AnswerE::E
Answer E Explanation	AnswerEExp::Absolute difference between both eGFRs should decrease. eGFR(creatinine) will be less of an overestimation of the true GFR. It will approach the GFR calculated by inulin clearance.
Right Answer	RightAnswer::C
Explanation	[[Explanation::Creatinine and use of creatinine clearance generally overestimates the estimated glomerular filtration rate (eGFR) due its combined filtration and secretion along the renal tubules. As such, inulin remains the gold standard for the estimation of the true GFR because it is only filtered and not secreted. Cimetidine, an H2 receptor blocker, has a role in blocking the tubular creatinine secretion. The use of cimetidine thus allow for only filtration of creatinine to occur by preventing its secretion. Thus, the estimation of GFR based on creatinine clearance is expected to become more accurate with the use of cimetidine. Because cimetidine blocks the tubular secretion of creatinine, the creatinine clearance is decreased because its secretion is decreased. Inulin clearance is not affected by this process. Finally, the originally overestimated eGFR (creatinine) will be reduced and eGFR(creatinine) will become approximately the same as eGFR(inulin), making the eGFR(creatinine) approximately reaching the true GFR. Educational Objective: Cimetidine blocks the tubular secretion of creatinine, thus decreasing creatinine clearance. As such, creatinine will only be filtered and approximation of eGFR based on creatinine values becomes more accurate and closer to the true GFR. Reference: Van Acker BA, Koomen GC, Koopman MG, et al. Creatinine clearance during cimetidine administration for measurement of glomerular filtration rate. Lancet. 1992;340(8831):1326-9. Educational Objective: References: ]]
Approved	Approved::No
Keyword	WBRKeyword::inulin, WBRKeyword::creatinine, WBRKeyword::secretion, WBRKeyword::cimetidine, WBRKeyword::filtration, WBRKeyword::eGFR, WBRKeyword::GFR, WBRKeyword::glomerular, WBRKeyword::filtration, WBRKeyword::rate, WBRKeyword::clearance, WBRKeyword::calculate, WBRKeyword::calculation, WBRKeyword::increase, WBRKeyword::decrease, WBRKeyword::change
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