Determinants of GFR US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Determinants of GFR. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Determinants of GFR 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
Determinants of GFR 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.
Determinants of GFR 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)
Determinants of GFR 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.
Determinants of GFR US Medical PG Question 3: On cardiology service rounds, your team sees a patient admitted with an acute congestive heart failure exacerbation. In congestive heart failure, decreased cardiac function leads to decreased renal perfusion, which eventually leads to excess volume retention. To test your knowledge of physiology, your attending asks you which segment of the nephron is responsible for the majority of water absorption. Which of the following is a correct pairing of the segment of the nephron that reabsorbs the majority of all filtered water with the means by which that segment absorbs water?
- A. Distal convoluted tubule via passive diffusion following ion reabsorption
- B. Distal convoluted tubule via aquaporin channels
- C. Thick ascending loop of Henle via passive diffusion following ion reabsorption
- D. Proximal convoluted tubule via passive diffusion following ion reabsorption (Correct Answer)
- E. Collecting duct via aquaporin channels
Determinants of GFR Explanation: ***Proximal convoluted tubule via passive diffusion following ion reabsorption***
- The **proximal convoluted tubule (PCT)** is responsible for reabsorbing approximately **65-70% of filtered water**, making it the primary site of water reabsorption in the nephron.
- This water reabsorption primarily occurs **passively**, following the active reabsorption of solutes (especially **sodium ions**), which creates an osmotic gradient.
*Distal convoluted tubule via passive diffusion following ion reabsorption*
- The **distal convoluted tubule (DCT)** reabsorbs a much smaller percentage of filtered water (around 5-10%) and its water reabsorption is largely **regulated by ADH**, not primarily simple passive diffusion following bulk ion reabsorption.
- While some passive water movement occurs, it is not the main mechanism or location for the majority of water reabsorption.
*Distal convoluted tubule via aquaporin channels*
- While aquaporin channels do play a role in water reabsorption in the DCT, particularly under the influence of **ADH**, the DCT is not the segment responsible for the **majority of all filtered water absorption**.
- The bulk of water reabsorption occurs earlier in the nephron, independently of ADH for the most part.
*Thick ascending loop of Henle via passive diffusion following ion reabsorption*
- The **thick ascending loop of Henle** is primarily involved in reabsorbing ions like Na+, K+, and Cl- but is largely **impermeable to water**.
- Its impermeability to water is crucial for creating the **osmotic gradient** in the renal medulla, which is necessary for later water reabsorption.
*Collecting duct via aquaporin channels*
- The **collecting duct** is critically important for **regulated water reabsorption** via **aquaporin-2 channels** under the influence of **ADH**, allowing for fine-tuning of urine concentration.
- However, it reabsorbs only a variable portion (typically 5-19%) of the remaining filtered water, not the **majority of all filtered water**.
Determinants of GFR US Medical PG Question 4: 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
Determinants of GFR 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.
Determinants of GFR US Medical PG Question 5: A 75-year-old woman is brought to a physician’s office by her son with complaints of diarrhea and vomiting for 1 day. Her stool is loose, watery, and yellow-colored, while her vomitus contains partially digested food particles. She denies having blood or mucus in her stools and vomitus. Since the onset of her symptoms, she has not had anything to eat and her son adds that she is unable to tolerate fluids. The past medical history is unremarkable and she does not take any medications regularly. The pulse is 115/min, the respiratory rate is 16/min, the blood pressure is 100/60 mm Hg, and the temperature is 37.0°C (98.6°F). The physical examination shows dry mucous membranes and slightly sunken eyes. The abdomen is soft and non-tender. Which of the following physiologic changes in glomerular filtration rate (GFR), renal plasma flow (RPF), and filtration fraction (FF) are expected?
- A. Decreased GFR, decreased RPF, decreased FF
- B. Decreased GFR, decreased RPF, no change in FF
- C. Increased GFR, increased RPF, increased FF
- D. Increased GFR, decreased RPF, increased FF
- E. Decreased GFR, decreased RPF, increased FF (Correct Answer)
Determinants of GFR Explanation: ***Decreased GFR, decreased RPF, increased FF***
- Due to **dehydration** from diarrhea and vomiting, there is a decrease in blood volume leading to decreased renal blood flow and **renal plasma flow (RPF)**.
- The body responds to hypovolemia by activating the renin-angiotensin-aldosterone system (RAAS) and sympathetic nervous system, which cause **preferential efferent arteriolar constriction** (more than afferent constriction). This helps maintain glomerular hydrostatic pressure despite reduced renal perfusion.
- As a result, **GFR decreases** but proportionally **less than RPF decreases**, causing the **filtration fraction (FF = GFR/RPF) to increase**.
- In this patient with significant dehydration (tachycardia, hypotension, dry mucous membranes), both GFR and RPF are reduced, but FF is elevated due to compensatory mechanisms.
*Decreased GFR, decreased RPF, decreased FF*
- While GFR and RPF will decrease due to dehydration, the **filtration fraction is expected to increase**, not decrease.
- A decreased FF would imply GFR fell proportionally more than RPF, which contradicts the physiologic response where efferent arteriolar constriction helps preserve GFR relative to RPF.
*Decreased GFR, decreased RPF, no change in FF*
- With significant fluid loss and compensatory mechanisms (efferent arteriolar constriction via angiotensin II), a change in **filtration fraction** is expected.
- The body actively alters arteriolar tone to prioritize GFR maintenance, which directly increases FF.
*Increased GFR, increased RPF, increased FF*
- This pattern suggests **hypervolemia** or increased renal perfusion, which directly contradicts the patient's severe dehydration.
- Both GFR and RPF are expected to decrease in volume depletion, not increase.
*Increased GFR, decreased RPF, increased FF*
- An increase in GFR is physiologically impossible given the patient's severe volume depletion and reduced renal perfusion.
- While FF does increase in dehydration, this occurs in the context of **both GFR and RPF being decreased**, not with an increased GFR.
Determinants of GFR US Medical PG Question 6: A 48-year-old woman comes to the physician for a follow-up examination. At her visit 1 month ago, her glomerular filtration rate (GFR) was 100 mL/min/1.73 m2 and her renal plasma flow (RPF) was 588 mL/min. Today, her RPF is 540 mL/min and her filtration fraction (FF) is 0.2. After her previous appointment, this patient was most likely started on a drug that has which of the following effects?
- A. Inhibition of the renal Na-K-Cl cotransporter
- B. Constriction of the afferent arteriole
- C. Relaxation of urinary smooth muscle
- D. Constriction of the efferent arteriole (Correct Answer)
- E. Inhibition of vasopressin
Determinants of GFR Explanation: ***Constriction of the efferent arteriole***
- The previous GFR was 100 mL/min and RPF was 588 mL/min. For the follow-up, RPF is 540 mL/min and FF is 0.2. The new GFR can be calculated as FF × RPF = 0.2 × 540 = **108 mL/min**.
- The patient shows **increased GFR** (100→108 mL/min) with **decreased RPF** (588→540 mL/min), resulting in an **increased filtration fraction**.
- Medications that **constrict the efferent arteriole**, such as **NSAIDs**, produce this pattern by blocking prostaglandin synthesis. Prostaglandins normally cause vasodilation (predominantly of the afferent arteriole). When blocked, there is relatively more **efferent arteriolar constriction**, which increases glomerular hydrostatic pressure, thereby **increasing GFR while reducing overall RPF**.
*Inhibition of the renal Na-K-Cl cotransporter*
- This effect describes **loop diuretics** (e.g., furosemide), which increase sodium excretion and water diuresis.
- Loop diuretics typically cause a **decrease in GFR** due to reduced fluid volume and lower filtration pressure, which contradicts the slight increase in GFR observed.
*Constriction of the afferent arteriole*
- **Afferent arteriole constriction** (e.g., by NSAIDs in high doses or norepinephrine) would decrease blood flow into the glomerulus, leading to a **decrease in both RPF and GFR**.
- While RPF decreased in this case, GFR actually increased, making this option incorrect.
*Relaxation of urinary smooth muscle*
- Relaxation of urinary smooth muscle is characteristic of drugs like **alpha-blockers** (e.g., tamsulosin) or antimuscarinics used for conditions like benign prostatic hyperplasia or overactive bladder.
- This effect primarily impacts urine flow out of the bladder and does **not directly affect GFR or RPF** in the way described.
*Inhibition of vasopressin*
- Vasopressin (ADH) inhibition leads to **increased water excretion** and is seen with drugs like **vasopressin receptor antagonists** (vaptans) or ethanol.
- While it affects fluid balance, it typically causes a **decrease in GFR** due to hypovolemia and has no direct mechanism to increase GFR with decreased RPF as observed.
Determinants of GFR US Medical PG Question 7: A 45-year-old man presents with a 3-day history of right-sided flank pain due to a lodged ureteral stone. What changes would be expected to be seen at the level of glomerular filtration?
- A. Increase in glomerular capillary oncotic pressure
- B. Increase in Bowman's space oncotic pressure
- C. Increase in filtration fraction
- D. Increase in Bowman's space hydrostatic pressure (Correct Answer)
- E. No change in filtration fraction
Determinants of GFR Explanation: ***Increase in Bowman's space hydrostatic pressure***
- A lodged ureteral stone causes **obstruction** of urine flow, leading to a backup of fluid in the renal tubules and eventually into **Bowman's space**.
- This increased fluid volume in Bowman's space directly raises its **hydrostatic pressure**, which opposes glomerular filtration, thereby reducing the net filtration pressure.
*Increase in glomerular capillary oncotic pressure*
- **Glomerular capillary oncotic pressure** primarily reflects the protein concentration within the glomerular capillaries, which would not be directly increased by a ureteral stone.
- This parameter typically rises when fluid is filtered out, increasing protein concentration in the remaining blood, but not as the initial insult from obstruction.
*Increase in Bowman's space oncotic pressure*
- **Bowman's space oncotic pressure** is normally very low because the glomerular filtration barrier prevents significant protein filtration.
- An increase in this pressure would imply increased protein leakage into Bowman's space, which is not a direct consequence of a ureteral obstruction.
*Increase in filtration fraction*
- The **filtration fraction** is the ratio of glomerular filtration rate (GFR) to renal plasma flow.
- Ureteral obstruction typically **decreases GFR** due to increased Bowman's space hydrostatic pressure, which would lead to a reduction, not an increase, in the filtration fraction, assuming renal plasma flow remains stable or slightly reduced.
*No change in filtration fraction*
- Ureteral obstruction significantly impacts the forces driving glomerular filtration, primarily by increasing **Bowman's space hydrostatic pressure**.
- This change inevitably leads to a **decrease in GFR**, thus altering the filtration fraction, meaning it would not remain unchanged.
Determinants of GFR US Medical PG Question 8: 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
Determinants of GFR 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.
Determinants of GFR US Medical PG 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
Determinants of GFR 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.
Determinants of GFR US Medical PG Question 10: A 69-year-old woman is admitted to the hospital with substernal, crushing chest pain. She is emergently moved to the cardiac catheterization lab where she undergoes cardiac angiography. Angiography reveals that the diameter of her left anterior descending artery (LAD) is 50% of normal. If her blood pressure, LAD length, and blood viscosity have not changed, which of the following represents the most likely change in LAD flow from baseline?
- A. Decreased by 93.75% (Correct Answer)
- B. Increased by 6.25%
- C. Decreased by 25%
- D. Decreased by 87.5%
- E. Increased by 25%
Determinants of GFR Explanation: ***Decreased by 93.75%***
- This option is correct based on Poiseuille's Law, which states that flow is proportional to the **fourth power of the radius (r^4)**. A 50% decrease in diameter means a 50% decrease in radius (0.5r).
- The new flow would be (0.5)^4 = 0.0625 times the original flow. Therefore, the decrease in flow is 1 - 0.0625 = 0.9375, or **93.75%**.
*Increased by 6.25%*
- This answer incorrectly suggests an **increase** in flow, which is contrary to the effect of a narrowed artery.
- While 6.25% represents the new flow as a percentage of baseline (since 0.0625 = 6.25%), the vessel stenosis causes a **decrease**, not an increase in flow.
*Decreased by 25%*
- This calculation might arise from considering a linear relationship (e.g., radius decreases by 50%, so flow decreases by 50% of 50%, which is incorrect).
- It does not account for the **fourth power relationship** between radius and flow according to Poiseuille's Law.
*Decreased by 87.5%*
- This percentage represents a calculation error, likely from misapplying the fourth power relationship or confusing the calculation steps.
- It does not accurately reflect the dramatic reduction in flow caused by a 50% reduction in vessel diameter.
*Increased by 25%*
- This option implies a significant increase in blood flow, which would not happen with a **stenosed artery**.
- It completely contradicts the physiological response to a **narrowed vessel**.
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