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?
Q2
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?
Q3
A 73-year-old male is brought in by ambulance after he was found to be lethargic and confused. He has not been routinely seeing a physician and is unable to recall how he came to be in the hospital. His temperature is 99°F (37°C), blood pressure is 150/95 mmHg, pulse is 75/min, and respirations are 18/min. His past medical history is significant for poorly controlled diabetes and longstanding hypertension, and he says that he has not been taking his medications recently. Labs are obtained and shown below:
Serum:
Na+: 142 mEq/L
Cl-: 105 mEq/L
K+: 5 mEq/L
HCO3-: 16 mEq/L
Urea nitrogen: 51 mg/dL
Glucose: 224 mg/dL
Creatinine: 2.6 mg/dL
Which of the following changes would most likely improve the abnormal parameter that is responsible for this patient's symptoms?
Q4
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?
Q5
An investigator is attempting to assess the glomerular filtration rate (GFR) of a healthy adult volunteer. The volunteer's inulin clearance is evaluated under continuous inulin infusion and urine collection and compared to the creatinine clearance. It is found that the estimated GFR based on the volunteer's creatinine clearance is 129 mL/min and the estimated GFR calculated using the inulin clearance is 122 mL/min. Which of the following is the best explanation for the difference in these measurements?
Q6
A researcher is studying the effects of a new antihypertensive medication on urine osmolality. She first measures urine osmolality in different parts of the nephron of a healthy human control. The findings are shown below:
Portion of nephron Urine osmolality (mOsmol/kg)
Proximal convoluted tubule 300
Loop of Henle, descending limb 1200
Loop of Henle, ascending limb 200
Distal convoluted tubule 100
Collecting duct 600
Which of the following is the most likely explanation for the urine osmolality in the ascending limb of the loop of Henle?
Q7
A 19-year-old man presents to the emergency department after a motor vehicle accident. The patient reports left shoulder pain that worsens with deep inspiration. Medical history is significant for a recent diagnosis of infectious mononucleosis. His temperature is 99°F (37.2°C), blood pressure is 80/55 mmHg, pulse is 115/min, and respiratory rate is 22/min. On physical exam, there is abdominal guarding, abdominal tenderness in the left upper quadrant, and rebound tenderness. The patient’s mucous membranes are dry and skin turgor is reduced. Which of the following most likely represents the acute changes in renal plasma flow (RPF) and glomerular filtration rate (GFR) in this patient?
Q8
To reduce the hemolysis that occurs with dialysis, researchers have developed an organic filtration membrane for dialysis that is believed to mimic the physiologic filtering apparatus of the human glomerulus. The permeability characteristics of this membrane are believed to be identical to those of the glomerular filtering membrane. Which of the following substances should be absent in the filtrate produced by this membrane?
Q9
A 55-year-old woman presents to a physician’s clinic for a diabetes follow-up. She recently lost weight and believes the diabetes is ‘winding down’ because the urinary frequency has slowed down compared to when her diabetes was "at its worst". She had been poorly compliant with medications, but she is now asking if she can decrease her medications as she feels like her diabetes is improving. Due to the decrease in urinary frequency, the physician is interested in interrogating her renal function. Which substance can be used to most accurately assess the glomerular filtration rate (GFR) in this patient?
Q10
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?
Glomerular filtration US Medical PG Practice Questions and MCQs
Question 1: 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
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**.
Question 2: 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
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.
Question 3: A 73-year-old male is brought in by ambulance after he was found to be lethargic and confused. He has not been routinely seeing a physician and is unable to recall how he came to be in the hospital. His temperature is 99°F (37°C), blood pressure is 150/95 mmHg, pulse is 75/min, and respirations are 18/min. His past medical history is significant for poorly controlled diabetes and longstanding hypertension, and he says that he has not been taking his medications recently. Labs are obtained and shown below:
Serum:
Na+: 142 mEq/L
Cl-: 105 mEq/L
K+: 5 mEq/L
HCO3-: 16 mEq/L
Urea nitrogen: 51 mg/dL
Glucose: 224 mg/dL
Creatinine: 2.6 mg/dL
Which of the following changes would most likely improve the abnormal parameter that is responsible for this patient's symptoms?
A. Increased Bowman's space hydrostatic pressure
B. Decreased filtration coefficient
C. Increased Bowman's space oncotic pressure
D. Decreased glomerular capillary hydrostatic pressure
E. Increased glomerular capillary hydrostatic pressure (Correct Answer)
Explanation: ***Increased glomerular capillary hydrostatic pressure***
- This patient presents with **acute kidney injury (AKI)** evidenced by **elevated creatinine (2.6 mg/dL)** and **BUN (51 mg/dL)**, causing uremic symptoms of **lethargy and confusion**
- The "abnormal parameter" is the **reduced GFR** causing azotemia and uremia
- To improve AKI and restore adequate filtration, **GFR must be increased**
- **Increasing glomerular capillary hydrostatic pressure** increases the net filtration pressure: **NFP = (PGC - PBS) - (πGC - πBS)**, where PGC is the primary driving force for filtration
- In prerenal AKI (likely in this patient with poor medication compliance for hypertension), restoring adequate renal perfusion pressure is the therapeutic goal
- While chronic hyperfiltration can contribute to long-term diabetic/hypertensive nephropathy, the **acute management priority** is restoring adequate GFR to clear uremic toxins
*Decreased glomerular capillary hydrostatic pressure*
- This would **decrease the net filtration pressure**, thereby **reducing GFR**
- Lower GFR would worsen azotemia and uremic symptoms
- This is the opposite of what's needed to improve acute kidney injury
*Increased Bowman's space hydrostatic pressure*
- This **opposes filtration** by increasing back-pressure against the glomerular capillaries
- Would **decrease GFR** and worsen the AKI
- Occurs pathologically in urinary tract obstruction
*Decreased filtration coefficient*
- The filtration coefficient (Kf) represents the permeability and surface area of the glomerular capillaries
- **Decreasing Kf reduces GFR**, worsening kidney function
- This represents glomerular damage, not a therapeutic intervention
*Increased Bowman's space oncotic pressure*
- This would theoretically **increase net filtration pressure** and GFR
- However, this is **physiologically implausible** as Bowman's space normally contains minimal protein (filtrate is protein-free)
- Significant protein in Bowman's space indicates severe glomerular damage with proteinuria, not a mechanism to improve function
Question 4: 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
E. Glucose: 148 mL/min, Sodium: 105 mL/min, Creatinine: 99 mL/min
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.
Question 5: An investigator is attempting to assess the glomerular filtration rate (GFR) of a healthy adult volunteer. The volunteer's inulin clearance is evaluated under continuous inulin infusion and urine collection and compared to the creatinine clearance. It is found that the estimated GFR based on the volunteer's creatinine clearance is 129 mL/min and the estimated GFR calculated using the inulin clearance is 122 mL/min. Which of the following is the best explanation for the difference in these measurements?
A. Inulin is actively secreted
B. Creatinine is not freely filtered
C. Inulin is not freely filtered
D. Creatinine is actively secreted (Correct Answer)
E. Creatinine is passively reabsorbed
Explanation: ***Creatinine is actively secreted***
- The higher estimated GFR by **creatinine clearance** (129 mL/min) compared to **inulin clearance** (122 mL/min) indicates that creatinine is not only filtered but also **secreted** by the renal tubules.
- This **active secretion** into the urine leads to a slightly higher amount of creatinine in the final urine than what would be present from filtration alone, thus overestimating the GFR.
*Inulin is actively secreted*
- **Inulin** is considered the **gold standard** for measuring GFR because it is **freely filtered** by the glomerulus, and is neither secreted nor reabsorbed by the renal tubules.
- If inulin were actively secreted, its clearance would be higher than the actual GFR, which is contrary to the observation of a lower inulin clearance compared to creatinine clearance.
*Creatinine is not freely filtered*
- **Creatinine** is largely **freely filtered** by the glomeruli due to its small molecular size and lack of protein binding.
- If creatinine were not freely filtered, its clearance would be lower than the actual GFR, which is contrary to the observed higher creatinine clearance.
*Inulin is not freely filtered*
- **Inulin** is a small polysaccharide that is **freely filtered** by the renal glomeruli without significant impedance.
- Its property of being freely filtered and neither secreted nor reabsorbed is precisely why it serves as the reference standard for GFR measurement.
*Creatinine is passively reabsorbed*
- While some substances are passively reabsorbed, **creatinine** primarily undergoes **filtration and active secretion**, with negligible or no passive reabsorption under normal physiological conditions.
- If creatinine were passively reabsorbed, its clearance would be lower than the actual GFR, leading to an underestimation, which is not what the data shows.
Question 6: A researcher is studying the effects of a new antihypertensive medication on urine osmolality. She first measures urine osmolality in different parts of the nephron of a healthy human control. The findings are shown below:
Portion of nephron Urine osmolality (mOsmol/kg)
Proximal convoluted tubule 300
Loop of Henle, descending limb 1200
Loop of Henle, ascending limb 200
Distal convoluted tubule 100
Collecting duct 600
Which of the following is the most likely explanation for the urine osmolality in the ascending limb of the loop of Henle?
A. Increased urea excretion
B. Increased transcription of water channels
C. Impermeability to water (Correct Answer)
D. Increased bicarbonate reabsorption
E. Impermeability to sodium
Explanation: ***Impermeability to water***
- The **ascending limb of the loop of Henle** is notable for its **water impermeability** due to the absence of aquaporins.
- This impermeability, coupled with active reabsorption of solutes, leads to the production of **hypoosmotic fluid** (200 mOsmol/kg) in this segment.
*Increased urea excretion*
- While urea is a major contributor to medullary osmolality and is excreted, it is primarily reabsorbed in the **collecting duct** and secreted into the loop of Henle, not directly explaining the low osmolality in the ascending limb.
- Increased urea excretion on its own would likely lead to a higher, not lower, osmolality of the urine exiting the kidney.
*Increased transcription of water channels*
- Increased transcription of water channels (**aquaporins**) would make the tubule more permeable to water, leading to water reabsorption and an **increase in osmolality**, which contradicts the observed hypoosmotic fluid.
- The ascending limb is primarily involved in **solute reabsorption** without water, making it dilute.
*Increased bicarbonate reabsorption*
- **Bicarbonate reabsorption** primarily occurs in the **proximal tubule** and is crucial for acid-base balance, not directly impacting the dramatic osmolality changes in the ascending limb.
- While some bicarbonate is reabsorbed in the ascending limb, it does not explain the significant decrease in fluid osmolality.
*Impermeability to sodium*
- The ascending limb is **highly permeable to sodium** and actively reabsorbs it via the **Na-K-2Cl cotransporter**, which is crucial for diluting the tubular fluid.
- If it were impermeable to sodium, the reabsorption of solutes would cease, and the osmolality would not decrease as observed.
Question 7: A 19-year-old man presents to the emergency department after a motor vehicle accident. The patient reports left shoulder pain that worsens with deep inspiration. Medical history is significant for a recent diagnosis of infectious mononucleosis. His temperature is 99°F (37.2°C), blood pressure is 80/55 mmHg, pulse is 115/min, and respiratory rate is 22/min. On physical exam, there is abdominal guarding, abdominal tenderness in the left upper quadrant, and rebound tenderness. The patient’s mucous membranes are dry and skin turgor is reduced. Which of the following most likely represents the acute changes in renal plasma flow (RPF) and glomerular filtration rate (GFR) in this patient?
A. No change in RPF and decreased GFR
B. Decreased RPF and decreased GFR (Correct Answer)
C. No change in RPF and increased GFR
D. Decreased RPF and no change in GFR
E. No change in RPF and GFR
Explanation: ***Decreased RPF and decreased GFR***
- This patient presents with signs of **hypovolemic shock** (hypotension with BP 80/55 mmHg, tachycardia, dry mucous membranes, reduced skin turgor) likely due to **splenic rupture** from the motor vehicle accident, exacerbated by splenomegaly from **infectious mononucleosis**.
- With a blood pressure of 80/55 mmHg (MAP ~73 mmHg), the patient is at or below the **lower limit of renal autoregulation** (~80 mmHg MAP).
- In acute hypovolemic shock, **renal blood flow and RPF decrease** due to systemic hypotension and **sympathetic vasoconstriction**.
- Although **angiotensin II-mediated efferent arteriolar constriction** attempts to preserve GFR by maintaining glomerular capillary pressure, this compensation is **insufficient** when MAP falls below the autoregulatory range.
- Result: **Both RPF and GFR decrease**, though GFR may be relatively preserved compared to the magnitude of RPF decrease initially.
*No change in RPF and decreased GFR*
- This scenario would suggest decreased filtration despite normal renal perfusion, implying a primary glomerular barrier problem.
- In hypovolemic shock, **RPF is always decreased** due to reduced systemic blood flow and renal vasoconstriction.
*Decreased RPF and no change in GFR*
- While renal autoregulation attempts to maintain stable GFR despite changes in blood pressure, this mechanism works only within the **autoregulatory range (MAP 80-180 mmHg)**.
- At BP 80/55 mmHg, autoregulation is overwhelmed, and **GFR will decrease** along with RPF.
*No change in RPF and increased GFR*
- An **increased GFR** is inconsistent with hypovolemic shock and would require either increased RPF or enhanced glomerular filtration pressure.
- Maintaining normal RPF during severe hypotension is physiologically implausible.
*No change in RPF and GFR*
- This suggests normal renal function despite **severe hypotension and hypovolemia**, which contradicts basic renal physiology.
- The body's compensatory mechanisms cannot fully maintain both RPF and GFR when systemic blood pressure falls below the autoregulatory threshold.
Question 8: To reduce the hemolysis that occurs with dialysis, researchers have developed an organic filtration membrane for dialysis that is believed to mimic the physiologic filtering apparatus of the human glomerulus. The permeability characteristics of this membrane are believed to be identical to those of the glomerular filtering membrane. Which of the following substances should be absent in the filtrate produced by this membrane?
A. Amino acids
B. Albumin (Correct Answer)
C. Urea
D. Creatinine
E. Sodium
Explanation: ***Albumin***
- The **glomerular filtration barrier** prevents passage of large proteins, especially **albumin (MW ~66 kDa)**, into the filtrate due to both **size selectivity** (fenestrated endothelium, basement membrane, slit diaphragms) and **charge selectivity** (negative charges repel anionic proteins).
- Albumin absence in filtrate indicates proper membrane function mimicking a healthy glomerulus.
- Normal albumin filtration is <30 mg/day; albuminuria indicates glomerular damage.
*Amino acids*
- Small molecules (MW ~100-200 Da) that are **freely filtered** by the glomerulus.
- Normally >95% reabsorbed in proximal tubule, but their presence in initial filtrate is expected and normal.
*Urea*
- Small waste product (MW 60 Da) that is **readily filtered** by the glomerulus.
- Primary uremic toxin removed during dialysis; freely passes through glomerular barrier.
*Creatinine*
- Small waste product (MW 113 Da) that is **freely filtered** and minimally reabsorbed.
- Used clinically to estimate GFR (creatinine clearance); must be present in filtrate.
*Sodium*
- Small cation (MW 23 Da) that is **freely filtered** by the glomerulus.
- Essential for fluid and electrolyte balance; normal presence in filtrate with subsequent tubular reabsorption (~99%).
Question 9: A 55-year-old woman presents to a physician’s clinic for a diabetes follow-up. She recently lost weight and believes the diabetes is ‘winding down’ because the urinary frequency has slowed down compared to when her diabetes was "at its worst". She had been poorly compliant with medications, but she is now asking if she can decrease her medications as she feels like her diabetes is improving. Due to the decrease in urinary frequency, the physician is interested in interrogating her renal function. Which substance can be used to most accurately assess the glomerular filtration rate (GFR) in this patient?
A. Para-aminohippurate (PAH)
B. Glucose
C. Inulin (Correct Answer)
D. Urea
E. Creatinine
Explanation: ***Correct Answer: Inulin***
- **Inulin** is freely filtered by the glomeruli and is neither reabsorbed nor secreted by the renal tubules, making its clearance rate an **accurate measure of GFR**.
- It is considered the **gold standard** for GFR measurement, although it is not routinely used in clinical practice due to its exogenous nature and the need for continuous infusion.
*Incorrect: Para-aminohippurate (PAH)*
- **PAH** is both filtered and actively secreted by the renal tubules, meaning its clearance reflects **renal plasma flow**, not GFR.
- Due to its high extraction fraction, it is used to measure **effective renal plasma flow (ERPF)**.
*Incorrect: Glucose*
- **Glucose** is freely filtered by the glomeruli but is almost completely reabsorbed in the proximal convoluted tubule in healthy individuals, especially at normal blood glucose levels.
- Therefore, glucose clearance is typically **zero** and does not measure GFR.
*Incorrect: Urea*
- **Urea** is filtered by the glomeruli, but a significant portion is **reabsorbed** by the renal tubules, particularly in states of lower urine flow.
- Its clearance **underestimates GFR** and varies with hydration status and protein intake, making it an unreliable sole measure of GFR.
*Incorrect: Creatinine*
- **Creatinine** is freely filtered by the glomeruli, but a small amount is also **secreted** by the renal tubules, leading to an overestimation of GFR, especially in advanced kidney disease.
- Although commonly used as an **estimate of GFR** in clinical practice due to its endogenous production, it is not as accurate as inulin.
Question 10: 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%
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.
