Q: A patient has a urine creatinine concentration ($$U) of 100 mg/dL$$, a urine flow rate ($$V) of 1 mL/$$min, and a plasma creatinine concentration ($$P) of 2 mg/dL. $$What is the renal plasma clearance of creatinine for this patient?

$$C = \frac{V \times U}{P} = \frac{1 \times 100}{2} = 50 mL/$$min

Q: If the glomerular filtration rate (GFR) is 120 mL/min and the PAH clearance is 625 mL/min, what percentage of renal plasma flow is filtered by the kidneys?

$$\frac{120}{625} \times 100 = 19.2\%$$

Question 1

Which of the following best describes the physiological significance of the glomerular filtration rate (GFR)?

Accepted Answer

It quantifies the rate at which the kidneys filter blood plasma, serving as a key indicator of renal function.

Question 2

A patient has a urine creatinine concentration ($$U) of 100 mg/dL$$, a urine flow rate ($$V) of 1 mL/$$min, and a plasma creatinine concentration ($$P) of 2 mg/dL. $$What is the renal plasma clearance of creatinine for this patient?

Accepted Answer

$$C = \frac{V 	imes U}{P} = \frac{1 	imes 100}{2} = 50 mL/$$min

Question 3

Which property makes creatinine a useful marker for estimating GFR in clinical practice?

Accepted Answer

It is produced in muscles at a relatively constant rate and is primarily cleared by glomerular filtration.

Question 4

Which equation is used to calculate renal plasma clearance ($$C) of a $$substance?

Accepted Answer

$$C = \frac{V 	imes U}{P}$$

Question 5

A patient’s serum creatinine is 1.2 mg/dL. According to the CKD-EPI equation, which factor could falsely elevate the estimated GFR (eGFR)?

Accepted Answer

Low muscle mass

Question 6

Why is para-aminohippuric acid (PAH) clearance used to estimate renal plasma flow rather than GFR?

Accepted Answer

PAH is filtered and almost completely secreted by the kidneys, so its clearance approximates total renal plasma flow.

Question 7

If a substance is filtered by the glomerulus, not reabsorbed, and not secreted, what will its renal plasma clearance be equal to?

Accepted Answer

It will be equal to the glomerular filtration rate (GFR).

Question 8

Which of the following best explains why PAH clearance is only accurate at low plasma concentrations?

Accepted Answer

Organic anion transporters become saturated at high PAH concentrations, limiting further secretion.

Question 9

A 10-year-old child has a serum creatinine of 0.5 mg/dL. According to the normal range table, is this value within the normal range for their age?

Accepted Answer

Yes, it is within the normal range (0.22–0.59 mg/dL).

Question 10

Which of the following scenarios would result in a renal plasma clearance greater than the GFR?

Accepted Answer

The substance is filtered and also secreted into the urine.

Question 11

A patient’s eGFR is calculated using the CKD-EPI equation. Which variable in the equation is most directly affected by muscle mass?

Accepted Answer

Serum creatinine ($$S_{cr})$$

Question 12

If the glomerular filtration rate (GFR) is 120 mL/min and the PAH clearance is 625 mL/min, what percentage of renal plasma flow is filtered by the kidneys?

Accepted Answer

$$\frac{120}{625} 	imes 100 = 19.2\%$$

Question 13

Which of the following is a limitation of using PAH clearance to estimate renal plasma flow?

Accepted Answer

PAH clearance is only accurate at low plasma concentrations due to transporter saturation.

Question 14

Which of the following would most likely cause a falsely low eGFR in a muscular individual?

Accepted Answer

Elevated serum creatinine due to increased muscle mass

Question 15

A drug inhibits the organic anion transporter responsible for PAH secretion. What effect would this have on PAH clearance?

Accepted Answer

PAH clearance would decrease, underestimating renal plasma flow.

Question 16

Which of the following best describes the fate of creatinine in the nephron?

Accepted Answer

It is filtered at the glomerulus and a small amount is secreted by the tubules.

Question 17

A patient’s urine contains a substance that is filtered, reabsorbed, and secreted. How does its renal plasma clearance compare to GFR?

Accepted Answer

It may be greater than, less than, or equal to GFR depending on the relative rates of reabsorption and secretion.

Question 18

Which of the following is NOT a factor that can affect estimated GFR (eGFR) calculations?

Accepted Answer

Blood pressure

Question 19

Which of the following best explains why inulin is considered the gold standard for measuring GFR?

Accepted Answer

Inulin is freely filtered, not reabsorbed, not secreted, and not metabolized by the kidneys.

P: Urinary System 4

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