An investigator is studying mechanisms of urea excretion in humans. During the experiment, a healthy male volunteer receives a continuous infusion of para-aminohippurate (PAH) to achieve a PAH plasma concentration of 0.01 mg/mL. A volume of 1.0 L of urine is collected over a period of 10 hours; the urine flow rate is 1.66 mL/min. The urinary concentration of PAH is measured to be 3.74 mg/mL and his serum concentration of urea is 0.2 mg/mL. Assuming a normal filtration fraction of 20%, which of the following best estimates the filtered load of urea in this patient?
Q12
A study of a new antihypertensive drug that affects glomerular filtration rate is being conducted. Infusion of drug X causes constriction of the efferent arteriole. After infusion of the drug, the following glomerular values are obtained from an experimental subject: hydrostatic pressure of the glomerular capillary (PGC) of 48 mm Hg, oncotic pressure of the glomerular capillary (πGC) of 23 mm Hg, hydrostatic pressure of Bowman’s space (PBS) of 10 mm Hg, and oncotic pressure of Bowman’s space (πBS) of 0 mm Hg. Which of the following best measures net filtration pressure in this participant?
Q13
A healthy 36-year-old Caucasian man takes part in an experimental drug trial. The drug is designed to lower glomerular filtration rate (GFR) while simultaneously raising the filtration fraction. Which of the following effects on the glomerulus would you expect the drug to have?
Q14
A 46-year-old woman with a history of type II diabetes mellitus is started on lisinopril for newly diagnosed hypertension by her primary care physician. At a follow-up appointment several weeks later, she reports decreased urine output, and she is noted to have generalized edema. Her creatinine is elevated compared to baseline. Given her presentation, which of the following changes in renal arteriolar blood flow and glomerular filtration rate (GFR) have likely occurred?
Q15
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?
Q16
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?
Glomerular filtration US Medical PG Practice Questions and MCQs
Question 11: An investigator is studying mechanisms of urea excretion in humans. During the experiment, a healthy male volunteer receives a continuous infusion of para-aminohippurate (PAH) to achieve a PAH plasma concentration of 0.01 mg/mL. A volume of 1.0 L of urine is collected over a period of 10 hours; the urine flow rate is 1.66 mL/min. The urinary concentration of PAH is measured to be 3.74 mg/mL and his serum concentration of urea is 0.2 mg/mL. Assuming a normal filtration fraction of 20%, which of the following best estimates the filtered load of urea in this patient?
A. 7 mg/min
B. 620 mg/min
C. 166 mg/min
D. 124 mg/min
E. 25 mg/min (Correct Answer)
Explanation: ***25 mg/min***
- The **filtered load of urea** is calculated by multiplying **GFR** by the **plasma concentration of urea**
- First, calculate **renal plasma flow (RPF)** using PAH clearance: RPF = (Urine flow rate × Urine PAH concentration) / Plasma PAH concentration = (1.66 mL/min × 3.74 mg/mL) / 0.01 mg/mL = **620.84 mL/min**
- Note: PAH is both filtered and secreted, so its clearance approximates **renal plasma flow**, not GFR
- Next, calculate **GFR** using the given filtration fraction: GFR = RPF × Filtration fraction = 620.84 mL/min × 0.20 = **124.17 mL/min**
- Finally, calculate **filtered load of urea**: Filtered load = GFR × Plasma urea concentration = 124.17 mL/min × 0.2 mg/mL = **24.83 mg/min ≈ 25 mg/min**
*124 mg/min*
- This represents the calculated **GFR**, not the filtered load of urea
- Filtered load requires multiplying GFR by the plasma concentration of the substance (urea)
- This is a common error when stopping the calculation one step too early
*620 mg/min*
- This represents the **renal plasma flow (RPF)** calculated from PAH clearance
- PAH clearance measures RPF because PAH is both filtered and secreted by the kidneys
- This is not the filtered load of urea, which requires using GFR (not RPF) multiplied by plasma urea concentration
*166 mg/min*
- This value results from incorrect calculation methodology
- Does not represent any physiologically relevant parameter in this problem
- May result from multiplying incorrect combinations of the given values
*7 mg/min*
- This value is too low and results from major calculation errors
- Does not correspond to any logical step in the proper calculation sequence
- May result from dividing instead of multiplying or using wrong values entirely
Question 12: A study of a new antihypertensive drug that affects glomerular filtration rate is being conducted. Infusion of drug X causes constriction of the efferent arteriole. After infusion of the drug, the following glomerular values are obtained from an experimental subject: hydrostatic pressure of the glomerular capillary (PGC) of 48 mm Hg, oncotic pressure of the glomerular capillary (πGC) of 23 mm Hg, hydrostatic pressure of Bowman’s space (PBS) of 10 mm Hg, and oncotic pressure of Bowman’s space (πBS) of 0 mm Hg. Which of the following best measures net filtration pressure in this participant?
A. 35 mm Hg
B. 0 mm Hg
C. 15 mm Hg (Correct Answer)
D. 61 mm Hg
E. 81 mm Hg
Explanation: ***15 mm Hg***
- The **net filtration pressure (NFP)** in the glomerulus is calculated using the formula: **NFP = (PGC - PBS) - (πGC - πBS)**.
- Plugging in the given values: NFP = (48 mm Hg - 10 mm Hg) - (23 mm Hg - 0 mm Hg) = 38 mm Hg - 23 mm Hg = **15 mm Hg**.
*35 mm Hg*
- This value would result if there was an error in applying the formula, such as adding PBS instead of subtracting it, or failing to subtract the oncotic pressure properly.
- For example: (48 - 23) + 10 = 25 + 10 = 35 mm Hg, which incorrectly adds the hydrostatic pressure of Bowman's space.
- This calculation does not correctly apply the formula for **net filtration pressure**, which requires subtracting both PBS and πGC from PGC.
*0 mm Hg*
- A net filtration pressure of **0 mm Hg** would indicate no net filtration is occurring, which is not the case given the provided pressure values.
- This would only happen if the favoring and opposing forces were exactly balanced, which they are not here.
*61 mm Hg*
- This value might arise from incorrectly adding the oncotic pressure instead of subtracting it.
- For example: (48 - 10) + 23 = 38 + 23 = 61 mm Hg, which incorrectly treats πGC as a favoring force rather than an opposing force.
*81 mm Hg*
- This value is significantly higher than any reasonable calculation of net filtration pressure with the given numbers.
- It would be obtained if all pressures were summed: 48 + 23 + 10 + 0 = 81 mm Hg, which completely ignores the directional nature of these forces.
Question 13: A healthy 36-year-old Caucasian man takes part in an experimental drug trial. The drug is designed to lower glomerular filtration rate (GFR) while simultaneously raising the filtration fraction. Which of the following effects on the glomerulus would you expect the drug to have?
A. Afferent arteriole dilation and efferent arteriole constriction
B. Afferent arteriole constriction and efferent arteriole vasodilation
C. Afferent arteriole dilation and efferent arteriole vasodilation
D. Increased oncotic pressure in Bowman's space
E. Afferent arteriole constriction and efferent arteriole constriction (Correct Answer)
Explanation: ***Afferent arteriole constriction and efferent arteriole constriction***
- **Afferent arteriole constriction** decreases renal plasma flow (RPF) significantly and reduces GFR.
- **Efferent arteriole constriction** increases glomerular capillary hydrostatic pressure (partially offsetting the GFR decrease) but further decreases RPF by increasing resistance to outflow.
- **Net effect**: GFR decreases (afferent effect dominates), but RPF decreases MORE than GFR, resulting in an **increased filtration fraction (FF = GFR/RPF)**.
- This is the classic mechanism of **angiotensin II**, which preferentially constricts the efferent arteriole to maintain GFR while reducing RPF, thereby increasing FF.
*Afferent arteriole dilation and efferent arteriole vasodilation*
- Both would **increase RPF** and allow more blood flow through the glomerulus.
- **Afferent dilation** would increase GFR, contradicting the desired effect.
- This combination would not achieve the goal of lowering GFR.
*Afferent arteriole dilation and efferent arteriole constriction*
- **Afferent dilation** increases blood flow into the glomerulus.
- **Efferent constriction** increases glomerular capillary pressure.
- Both effects would **increase GFR**, directly contradicting the drug's purpose.
- This is the mechanism that **increases** both GFR and FF, opposite of what's needed.
*Afferent arteriole constriction and efferent arteriole vasodilation*
- **Afferent constriction** decreases RPF and GFR.
- **Efferent vasodilation** decreases glomerular capillary pressure, further reducing GFR, but increases RPF.
- Both actions lower glomerular capillary pressure dramatically, causing GFR to fall.
- The net effect would **decrease FF** because GFR falls more than RPF falls (or RPF may even increase from efferent dilation).
*Increased oncotic pressure in Bowman's space*
- Increased oncotic pressure in Bowman's space would oppose filtration and reduce GFR.
- However, this would not affect RPF directly, and the effect on FF would be to decrease it (as GFR falls without a proportional change in RPF).
- This represents pathology (proteinuria/glomerular damage) rather than a typical pharmacologic mechanism for regulating filtration.
Question 14: A 46-year-old woman with a history of type II diabetes mellitus is started on lisinopril for newly diagnosed hypertension by her primary care physician. At a follow-up appointment several weeks later, she reports decreased urine output, and she is noted to have generalized edema. Her creatinine is elevated compared to baseline. Given her presentation, which of the following changes in renal arteriolar blood flow and glomerular filtration rate (GFR) have likely occurred?
A. Renal efferent arteriole vasodilation; decreased GFR (Correct Answer)
B. Renal afferent arteriole vasoconstriction; decreased GFR
C. Renal efferent arteriole vasodilation; no change in GFR
D. Renal afferent arteriole vasodilation; increased GFR
E. Renal efferent arteriole vasoconstriction; increased GFR
Explanation: ***Renal efferent arteriole vasodilation; decreased GFR***
- Lisinopril is an **ACE inhibitor** that blocks conversion of angiotensin I to angiotensin II
- Angiotensin II normally **preferentially constricts the efferent arteriole** to maintain glomerular filtration pressure
- Blocking angiotensin II causes **efferent arteriolar vasodilation**, reducing the pressure gradient across the glomerulus
- This decreases **intraglomerular hydrostatic pressure** and consequently **GFR**
- The clinical presentation (**elevated creatinine, decreased urine output, edema**) confirms decreased GFR
- This complication is particularly common in patients with **bilateral renal artery stenosis** or underlying renal hypoperfusion (as may occur in diabetic nephropathy)
*Renal afferent arteriole vasoconstriction; decreased GFR*
- While this would decrease GFR, **ACE inhibitors primarily affect the efferent arteriole**, not the afferent arteriole
- Angiotensin II has a **much greater effect on efferent** than afferent arterioles
- Afferent vasoconstriction would more likely result from NSAIDs, hypoperfusion, or increased sympathetic tone
*Renal efferent arteriole vasodilation; no change in GFR*
- Efferent arteriolar vasodilation **inherently decreases GFR** by reducing the glomerular pressure gradient
- The patient's **elevated creatinine and decreased urine output** directly contradict "no change in GFR"
- These clinical findings indicate significant reduction in renal function
*Renal afferent arteriole vasodilation; increased GFR*
- **ACE inhibitors do not cause afferent arteriole vasodilation**; their primary mechanism is efferent arteriolar dilation
- Increased GFR would lead to **increased urine output**, not the decreased output observed
- This does not explain the **elevated creatinine** or edema
*Renal efferent arteriole vasoconstriction; increased GFR*
- This is the **opposite of ACE inhibitor mechanism** - lisinopril prevents efferent vasoconstriction by blocking angiotensin II
- Efferent vasoconstriction is what angiotensin II normally does to **maintain GFR**, which ACE inhibitors prevent
- This would not explain the patient's **decreased renal function**
Question 15: 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)
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.
Question 16: 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
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.
