GFR measurement methods US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for GFR measurement methods. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
GFR measurement methods US Medical PG Question 1: Which factor most strongly influences protein filtration at the glomerulus?
- A. Electrical charge
- B. Molecular size (Correct Answer)
- C. Shape
- D. Temperature
GFR measurement methods Explanation: ***Molecular size***
- The glomerular filtration barrier, particularly the **slit diaphragms** between podocytes, acts as a size-selective filter, restricting the passage of larger molecules.
- Proteins like **albumin** (molecular radius ~36 Å, molecular weight ~69 kDa) are significantly large, making them difficult to pass through the filtration barrier.
- Size selectivity is the **primary and most important** factor in protein filtration.
*Electrical charge*
- The glomerular basement membrane contains **negatively charged proteoglycans** (heparan sulfate), which repel negatively charged proteins like albumin, contributing to their retention.
- While important, the role of electrical charge is **secondary** to molecular size in preventing the bulk passage of most proteins.
*Shape*
- While abnormal protein shapes (e.g., **amyloid fibrils**) can impact filtration in specific disease states, the typical physiological filtration of most proteins is primarily governed by size and charge.
- The inherent shape of normal globular proteins plays a less direct role compared to their overall size.
*Temperature*
- **Physiological temperature** is relatively constant in the body and does not directly influence the molecular interactions and physical properties of the glomerular filtration barrier in a way that significantly alters protein filtration.
- Temperature changes would lead to denaturation or aggregation, which are not the primary determinants of normal protein filtration.
GFR measurement methods US Medical PG Question 2: A healthy 30-year-old female has a measured creatinine clearance of 100 mL/min. She has a filtration fraction (FF) of 25%. Serum analysis reveals a creatinine level of 0.9 mg/dL and an elevated hematocrit of 0.6. Which of the following is the best estimate of this patient’s renal blood flow?
- A. 1.2 L/min
- B. 600 mL/min
- C. 800 mL/min
- D. 400 mL/min
- E. 1.0 L/min (Correct Answer)
GFR measurement methods Explanation: ***1.0 L/min***
- The **renal plasma flow (RPF)** can be calculated by dividing the **creatinine clearance (which approximates GFR)** by the **filtration fraction (FF)**: RPF = GFR / FF = 100 mL/min / 0.25 = 400 mL/min.
- To find the **renal blood flow (RBF)**, we use the formula RBF = RPF / (1 - Hematocrit). Given RPF = 400 mL/min and Hematocrit = 0.6, RBF = 400 mL/min / (1 - 0.6) = 400 mL/min / 0.4 = 1000 mL/min, or **1.0 L/min**.
*1.2 L/min*
- This value would result if the hematocrit were lower (e.g., 0.5) or if the GFR or FF were different, leading to an incorrect RPF or RBF calculation.
- It does not align with the provided values when applying the standard physiological formulas relating GFR, FF, RPF, and hematocrit.
*600 mL/min*
- This value might be obtained if the hematocrit was significantly underestimated or if the RPF calculation was incorrect in determining the RBF.
- It arises from using an incorrect formula or misinterpreting the relationship between plasma flow and blood flow.
*800 mL/min*
- This result would occur if the calculation for RPF or the subsequent RBF was erroneous, possibly by using an incorrect denominator in the RBF formula.
- For example, if RPF was incorrectly assumed to be 320 mL/min and divided by 0.4 (1-Hematocrit).
*400 mL/min*
- This value represents the calculated **renal plasma flow (RPF)**, not the **renal blood flow (RBF)**.
- RBF is always higher than RPF because it includes both plasma and cellular components of blood.
GFR measurement methods US Medical PG Question 3: An investigator is studying the effect of antihypertensive drugs on cardiac output and renal blood flow. For comparison, a healthy volunteer is given a placebo and a continuous infusion of para-aminohippuric acid (PAH) to achieve a plasma concentration of 0.02 mg/ml. His urinary flow rate is 1.5 ml/min and the urinary concentration of PAH is measured to be 8 mg/ml. His hematocrit is 50%. Which of the following values best estimates cardiac output in this volunteer?
- A. 8 L/min
- B. 3 L/min
- C. 4 L/min
- D. 1.2 L/min
- E. 6 L/min (Correct Answer)
GFR measurement methods Explanation: ***6 L/min***
- This value represents the estimated **cardiac output** based on the calculated renal blood flow.
- Step 1: Calculate renal plasma flow (RPF) using PAH clearance: RPF = (Urinary PAH × Urine flow rate) / Plasma PAH = (8 mg/ml × 1.5 ml/min) / 0.02 mg/ml = 600 ml/min = 0.6 L/min
- Step 2: Calculate renal blood flow (RBF): Since hematocrit is 50%, RBF = RPF / (1 - Hematocrit) = 0.6 / 0.5 = 1.2 L/min
- Step 3: Estimate cardiac output: The kidneys normally receive approximately **20-25% of cardiac output**. Using 20%: Cardiac Output = RBF / 0.20 = 1.2 / 0.20 = **6 L/min**
- This is consistent with normal resting cardiac output in a healthy adult.
*8 L/min*
- This value overestimates cardiac output based on the renal blood flow calculation.
- While some individuals may have higher cardiac output during exercise, the calculated RBF of 1.2 L/min suggests a resting cardiac output closer to 6 L/min.
*3 L/min*
- This value significantly underestimates cardiac output.
- If cardiac output were 3 L/min, the kidneys would be receiving 40% of cardiac output (1.2/3), which is physiologically implausible at rest.
*4 L/min*
- This value underestimates cardiac output based on the renal data.
- This would mean kidneys receive 30% of cardiac output (1.2/4), which is higher than the typical 20-25%.
*1.2 L/min*
- This is the calculated **renal blood flow**, not cardiac output.
- While this calculation is correct for RBF, the question specifically asks for cardiac output estimation, which requires accounting for the fact that kidneys receive only about 20-25% of total cardiac output.
GFR measurement methods US Medical PG Question 4: A 70-year-old female with chronic kidney failure secondary to diabetes asks her nephrologist to educate her about the techniques used to evaluate the degree of kidney failure progression. She learns about the concept of glomerular filtration rate (GFR) and learns that it can be estimated by measuring the levels of some substances. The clearance of which of the following substances is the most accurate estimate for GFR?
- A. Paraaminohippurate (PAH)
- B. Sodium
- C. Inulin (Correct Answer)
- D. Creatinine
- E. Glucose
GFR measurement methods Explanation: ***Inulin***
- **Inulin** is freely filtered by the glomeruli and is neither reabsorbed nor secreted by the renal tubules, making its clearance the **gold standard** for accurately measuring GFR.
- Due to its ideal physiological properties, inulin clearance perfectly reflects the rate at which plasma is filtered by the kidneys.
*Paraaminohippurate (PAH)*
- **PAH** is almost completely cleared from the blood by both glomerular filtration and **tubular secretion**, making its clearance an accurate measure of **renal plasma flow (RPF)**, not GFR.
- While important for assessing renal blood flow, it does not directly reflect the filtration capacity of the glomeruli.
*Sodium*
- **Sodium** is freely filtered at the glomerulus, but a significant portion (approximately **99%**) is **reabsorbed** by the renal tubules.
- Its clearance is highly variable and depends on various physiological factors, making it unsuitable for GFR estimation.
*Creatinine*
- **Creatinine** is freely filtered by the glomeruli and is also **modestly secreted** by the renal tubules, leading to an **overestimation of GFR** at lower kidney function levels.
- Despite being the most commonly used clinical marker due to its endogenous production, its tubular secretion makes it less accurate than inulin.
*Glucose*
- **Glucose** is freely filtered by the glomeruli but is almost **completely reabsorbed** by the renal tubules under normal physiological conditions.
- Its presence in urine (glycosuria) indicates a high plasma glucose level or tubular reabsorption defects, not a measure of GFR.
GFR measurement methods US Medical PG Question 5: A large pharmaceutical company is seeking healthy volunteers to participate in a drug trial. The drug is excreted in the urine, and the volunteers must agree to laboratory testing before enrolling in the trial.
The laboratory results of one volunteer are shown below:
Serum glucose (random) 148 mg/dL
Sodium 140 mEq/L
Potassium 4 mEq/L
Chloride 100 mEq/L
Serum creatinine 1 mg/dL
Urinalysis test results:
Glucose absent
Sodium 35 mEq/L
Potassium 10 mEq/L
Chloride 45 mEq/L
Creatinine 100 mg/dL
Assuming a urine flow rate of 1 mL/min, which set of values below is the clearance of glucose, sodium, and creatinine in this patient?
- A. Glucose: 0 mL/min, Sodium: 45 mL/min, Creatinine: 100 mL/min
- B. Glucose: 0 mL/min, Sodium: 4 mL/min, Creatinine: 0.01 mL/min
- C. Glucose: 0 mL/min, Sodium: 48 mL/min, Creatinine: 100 mL/min
- D. Glucose: 0 mL/min, Sodium: 0.25 mL/min, Creatinine: 100 mL/min (Correct Answer)
- E. Glucose: 148 mL/min, Sodium: 105 mL/min, Creatinine: 99 mL/min
GFR measurement methods Explanation: ***Glucose: 0 mg/dL, Sodium: 0.25 mL/min, Creatinine: 100 mL/min***
- **Glucose clearance**: Since urine glucose is absent despite a random serum glucose of 148 mg/dL, it indicates **complete reabsorption** of filtered glucose, resulting in a clearance of 0.
- **Sodium clearance**: Calculated as (Urine Na * Urine Flow Rate) / Serum Na = (35 mEq/L * 1 mL/min) / 140 mEq/L = **0.25 mL/min**.
- **Creatinine clearance**: Calculated as (Urine Creatinine * Urine Flow Rate) / Serum Creatinine = (100 mg/dL * 1 mL/min) / 1 mg/dL = **100 mL/min**.
*Glucose: 0 mL/min, Sodium: 45 mL/min, Creatinine: 100 mL/min*
- This option correctly identifies **glucose clearance as 0** and **creatinine clearance as 100 mL/min**.
- However, the **sodium clearance calculation is incorrect**; 45 mEq/L is simply the urine sodium concentration, not the clearance value.
*Glucose: 0 mL/min, Sodium: 4 mL/min, Creatinine: 0.01 mL/min*
- While **glucose clearance is correctly identified as 0**, both **sodium and creatinine clearances are incorrect**.
- Sodium clearance is 0.25 mL/min, and creatinine clearance is 100 mL/min, making these values significantly underestimated.
*Glucose: 0 mL/min, Sodium: 48 mL/min, Creatinine: 100 mL/min*
- This option correctly identifies **glucose clearance as 0** and **creatinine clearance as 100 mL/min**.
- The **sodium clearance calculation is incorrect**; the value 48 mL/min does not correspond to the given data.
*Glucose: 148 mL/min, Sodium: 105 mL/min, Creatinine: 99 mL/min*
- This option is incorrect because **glucose clearance is 0**, not 148 mL/min, as glucose is completely reabsorbed.
- The calculated values for **sodium and creatinine clearance are also incorrect** based on the provided data and formulas.
GFR measurement methods US Medical PG Question 6: A researcher is investigating the effects of a new antihypertensive medication on renal physiology. She gives a subject a dose of the new medication, and she then collects plasma and urine samples. She finds the following: Hematocrit: 40%; Serum creatinine: 0.0125 mg/mL; Urine creatinine: 1.25 mg/mL. Urinary output is 1 mL/min. Renal blood flow is 1 L/min. Based on the above information and approximating that the creatinine clearance is equal to the GFR, what answer best approximates filtration fraction in this case?
- A. 10%
- B. 17% (Correct Answer)
- C. 33%
- D. 50%
- E. 25%
GFR measurement methods Explanation: ***17%***
- First, calculate **GFR** using the creatinine clearance formula: GFR = (Urine creatinine × Urinary output) / Serum creatinine = (1.25 mg/mL × 1 mL/min) / 0.0125 mg/mL = **100 mL/min**.
- Next, calculate **Renal Plasma Flow (RPF)** from Renal Blood Flow (RBF) and Hematocrit: RPF = RBF × (1 - Hematocrit) = 1000 mL/min × (1 - 0.40) = **600 mL/min**.
- Finally, calculate **Filtration Fraction (FF)** = GFR / RPF = 100 mL/min / 600 mL/min = 0.1667 = **16.7%, which approximates to 17%**.
- This is the correct answer based on the physiological calculations and represents a normal filtration fraction.
*10%*
- This would correspond to a filtration fraction of 0.10, which would require either a GFR of 60 mL/min (lower than calculated) or an RPF of 1000 mL/min (higher than calculated).
- This value is too low given the provided parameters and doesn't match the calculation from the given data.
*25%*
- This value would suggest FF = 0.25, requiring a GFR of 150 mL/min with the calculated RPF of 600 mL/min.
- This is higher than the calculated GFR of 100 mL/min and doesn't match the given creatinine values.
*33%*
- This would imply FF = 0.33, requiring a GFR of approximately 200 mL/min with RPF of 600 mL/min.
- This is significantly higher than the calculated GFR and would represent an abnormally elevated filtration fraction.
*50%*
- A filtration fraction of 50% is unphysiologically high and would indicate severe pathology.
- This would require a GFR of 300 mL/min with the calculated RPF, which is impossible given the provided creatinine clearance data.
GFR measurement methods US Medical PG Question 7: A 58-year-old Caucasian woman visits her primary care physician for an annual check-up. She has a history of type 2 diabetes mellitus and stage 3A chronic kidney disease. Her estimated glomerular filtration rate has not changed since her last visit. Today, her parathyroid levels are moderately elevated. She lives at home with her husband and 2 children and works as a bank clerk. Her vitals are normal, and her physical examination is unremarkable. Which of the following explains this new finding?
- A. Uremia
- B. Acidemia
- C. Hyperuricemia
- D. Hypercalcemia
- E. Phosphate retention (Correct Answer)
GFR measurement methods Explanation: ***Phosphate retention***
- **Chronic kidney disease** often leads to **phosphate retention** because the damaged kidneys cannot effectively excrete phosphate.
- This elevated phosphate stimulates the parathyroid glands to secrete more **parathyroid hormone (PTH)** as a compensatory mechanism, leading to secondary hyperparathyroidism.
*Uremia*
- While uremia (accumulation of nitrogenous waste products) is a feature of chronic kidney disease, it is not the **direct cause** of elevated parathyroid levels.
- Uremia primarily causes symptoms like fatigue, nausea, and altered mental status, but it doesn't independently trigger PTH release in the same direct manner as phosphate retention or hypocalcemia.
*Acidemia*
- **Metabolic acidosis** is common in chronic kidney disease, but it generally **inhibits** PTH secretion, not stimulates it.
- While it can worsen bone disease, acidemia itself does not explain the primary elevation of parathyroid hormone.
*Hyperuricemia*
- **Hyperuricemia** (elevated uric acid levels) is often associated with chronic kidney disease due to decreased renal excretion of uric acid.
- However, hyperuricemia does not directly cause or explain elevated parathyroid hormone levels.
*Hypercalcemia*
- **Hypercalcemia** would typically **suppress** parathyroid hormone secretion, not elevate it.
- In chronic kidney disease, **hypocalcemia** (due to impaired vitamin D activation and phosphate retention) is more common and would stimulate PTH.
GFR measurement methods US Medical PG Question 8: A 68-year-old woman presents to her primary care physician for a regular check-up. She complains of swelling of her legs and face, which is worse in the morning and decreases during the day. She was diagnosed with type 2 diabetes mellitus a year ago and prescribed metformin, but she has not been compliant with it preferring 'natural remedies' over the medications. She does not have a history of cardiovascular disease or malignancy. Her vital signs are as follows: blood pressure measured on the right hand is 130/85 mm Hg, on the left hand, is 110/80 mm Hg, heart rate is 79/min, respiratory rate is 16/min, and the temperature is 36.6℃ (97.9°F). Physical examination reveals S1 accentuation best heard in the second intercostal space at the right sternal border. Facial and lower limbs edema are evident. The results of the laboratory tests are shown in the table below.
Fasting plasma glucose 164 mg/dL
HbA1c 10.4%
Total cholesterol 243.2 mg/dL
Triglycerides 194.7 mg/dL
Creatinine 1.8 mg/dL
Urea nitrogen 22.4 mg/dL
Ca2+ 9.6 mg/dL
PO42- 3.84 mg/dL
Which of the following statements best describes this patient's condition?
- A. There is an error in Ca2+ measurement because the level of serum calcium is always decreased in the patient’s condition.
- B. If measured in this patient, there would be an increased PTH level. (Correct Answer)
- C. Increase in 1α, 25(OH)2D3 production is likely to contribute to alteration of the patient’s laboratory values.
- D. The calcitriol level is unlikely to be affected in this patient.
- E. Hypoparathyroidism is most likely the cause of the patient’s altered laboratory results.
GFR measurement methods Explanation: ***If measured in this patient, there would be an increased PTH level.***
- This patient presents with signs of **chronic kidney disease (CKD)**, indicated by **elevated creatinine (1.8 mg/dL)** and **urea nitrogen (22.4 mg/dL)**, along with edema.
- In CKD, the kidneys are less able to excrete phosphate and synthesize calcitriol (active vitamin D), leading to **hyperphosphatemia (PO42- 3.84 mg/dL)** and **hypocalcemia**. These imbalances stimulate the parathyroid glands to produce more **parathyroid hormone (PTH)** as a compensatory mechanism, a condition known as **secondary hyperparathyroidism**.
*There is an error in Ca2+ measurement because the level of serum calcium is always decreased in the patient's condition.*
- While **hypocalcemia** is common in CKD, it's not universally present, especially in early or moderate stages.
- The measured **calcium level (9.6 mg/dL)** is within the normal range, suggesting that the compensatory increase in **PTH** might be maintaining **normocalcemia** or that severe hypocalcemia has not yet developed.
*Increase in 1α, 25(OH)2D3 production is likely to contribute to alteration of the patient's laboratory values.*
- In CKD, there is a **decreased production of 1α,25(OH)2D3 (calcitriol)** by the kidneys, not an increase.
- The enzyme **1-alpha-hydroxylase**, responsible for converting 25-hydroxyvitamin D to active calcitriol, becomes deficient as renal function declines.
*The calcitriol level is unlikely to be affected in this patient.*
- The **calcitriol level is significantly affected in CKD**, specifically it is reduced.
- Reduced calcitriol synthesis is a key factor in the development of **secondary hyperparathyroidism** and **renal osteodystrophy**.
*Hypoparathyroidism is most likely the cause of the patient's altered laboratory results.*
- **Hypoparathyroidism** would lead to **low PTH levels**, typically resulting in **hypocalcemia** and **hyperphosphatemia** due to impaired renal phosphate excretion.
- This patient's presentation, particularly the high phosphate and normal calcium (suggesting compensation), is consistent with **hyperparathyroidism secondary to chronic kidney disease**, not hypoparathyroidism.
GFR measurement methods US Medical PG Question 9: A 9-year-old boy is brought to the physician's office by his mother because of facial swelling for the past 2 days. The mother says that her son has always been healthy and active but is becoming increasingly lethargic and now has a puffy face. Upon inquiry, the boy describes a foamy appearance of his urine, but denies having blood in the urine, urinary frequency at night, or pain during urination. He has no history of renal or urinary diseases. Physical examination is unremarkable, except for generalized swelling of the face and pitting edema on the lower limbs. Dipstick analysis reveals 4+ proteinuria. An abdominal ultrasound shows normal kidney size and morphology. A renal biopsy yields no findings under light and fluorescence microscopy; however, glomerular podocyte foot effacement is noted on electron microscopy. Which of the following changes in Starling forces occurs in this patient's condition?
- A. Decreased oncotic pressure in the Bowman's capsule
- B. Increased hydrostatic pressure in the Bowman's capsule
- C. Decreased hydrostatic pressure in the Bowman's capsule
- D. Decreased glomerular oncotic pressure (Correct Answer)
- E. Increased glomerular hydrostatic pressure
GFR measurement methods Explanation: ***Decreased glomerular oncotic pressure***
- The patient presents with **nephrotic syndrome**, characterized by severe proteinuria (4+ on dipstick), edema, and **minimal change disease** (podocyte foot effacement on electron microscopy without changes on light or fluorescence microscopy).
- In nephrotic syndrome, large amounts of plasma proteins, particularly **albumin**, are lost in the urine, leading to **hypoalbuminemia** and a significant decrease in the **oncotic pressure of the plasma** (and thus the glomerular capillaries).
*Decreased oncotic pressure in the Bowman's capsule*
- The Bowman's capsule normally has a **very low oncotic pressure** due to the almost complete absence of proteins in the filtrate.
- While theoretically a massive increase in protein filtration could increase it, the primary Starling force affected by protein loss in nephrotic syndrome is the **plasma oncotic pressure**.
*Increased hydrostatic pressure in the Bowman's capsule*
- This condition is not typically associated with nephrotic syndrome and would rather **impair filtration**.
- Increased hydrostatic pressure in the Bowman's capsule is usually seen in conditions causing **urinary tract obstruction**, which is not present here.
*Decreased hydrostatic pressure in the Bowman's capsule*
- This would tend to **increase glomerular filtration rate** by favoring filtration, which is not the primary physiological change driving edema in nephrotic syndrome.
- There is no clinical indication for such a change in this patient's presentation.
*Increased glomerular hydrostatic pressure*
- While sometimes seen in specific glomerular diseases, this is not the primary or defining Starling force change in nephrotic syndrome leading to systemic edema.
- Increased glomerular hydrostatic pressure would tend to **increase filtration**, potentially worsening proteinuria, but the fundamental issue in nephrotic syndrome is the **loss of oncotic pressure due to protein leakage**.
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