WBR234

Author	[[PageAuthor::Rim Halaby, M.D. [1] (Reviewed by Will Gibson)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Physiology
Sub Category	SubCategory::Renal
Prompt	[[Prompt::An experiment to measure the filtration fraction in healthy volunteers is conducted. One sample of the available urine and serum values is shown below. If the urine flow is 2.0 ml/min, what is the best approximate filtration fraction? ]]
Answer A	AnswerA::20%
Answer A Explanation	AnswerAExp::One could obtain this number by using the ratio of the serum PAH to serum creatinine. However, this does not reflect any standard physiologic parameter.
Answer B	AnswerB::30%
Answer B Explanation	AnswerBExp::According to the calculation, this is an incorrect answer.
Answer C	AnswerC::35%
Answer C Explanation	AnswerCExp::According to the calculation, this is an incorrect answer.
Answer D	AnswerD::40%
Answer D Explanation	AnswerDExp::According to the calculation, this is a correct answer.
Answer E	AnswerE::100%
Answer E Explanation	AnswerEExp::This number could be achieved by dividing the renal creatinine concentration by the serum creatinine concentration. However, this measure does not reflect any normal physiologic parameter.
Right Answer	RightAnswer::D
Explanation	[[Explanation::Despite the presence of several parameters, utilization of only creatinine and PAH is required to calculate the filtration fraction. Filtration fraction (FF) is calculated by the following equation: FF (%) = (GFR / RPF) x 100 GFR = (UCr x V)/PCr = (100 mg/dL x 2 ml/min ) / (1 mg/dL) = 200 mL/min RPF = (UPAH x V)/PPAH = (50 mg/dL x 2 ml/min ) / (0.2 mg/dL) = 500 mL/min FF (%) = (GFR / RPF) x 100 = (200/500) x 100 = 0.4 x 100 = 40% Abbreviations: GFR: Glomerular Filtration Rate (mL/min) UCr: Urinary creatinine concentration (mg/mL) V: Volume of urine produced per unit time (mL/min) PCr: Plasma creatinine concentration (mg/mL) RPF: Renal plasma flow UPAH: Urinary Para-aminohippurate (PAH) concentration (mg/mL) PPAH: Plasma Para-aminohippurate (PAH) concentration (mg/mL) FF: Filtration fraction Educational Objective: The filtration fraction can be calculated by the following formula: FF (%) = (GFR/RPF) x 100 References: First Aid 2015 page 530]]
Approved	Approved::Yes
Keyword	WBRKeyword::Renal physiology, WBRKeyword::Renal plasma flow, WBRKeyword::Filtration fraction, WBRKeyword::Nephrology, WBRKeyword::Kidney, WBRKeyword::GFR, WBRKeyword::PAH, WBRKeyword::Creatinine
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