Proximal tubule reabsorption US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Proximal tubule reabsorption. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Proximal tubule reabsorption US Medical PG Question 1: A scientist is studying the excretion of a novel toxin X by the kidney in order to understand the dynamics of this new substance. He discovers that this new toxin X has a clearance that is half that of inulin in a particular patient. This patient's filtration fraction is 20% and his para-aminohippuric acid (PAH) dynamics are as follows:
Urine volume: 100 mL/min
Urine PAH concentration: 30 mg/mL
Plasma PAH concentration: 5 mg/mL
Given these findings, what is the clearance of the novel toxin X?
- A. 1,500 mL/min
- B. 600 mL/min
- C. 300 mL/min
- D. 60 mL/min (Correct Answer)
- E. 120 mL/min
Proximal tubule reabsorption Explanation: ***60 ml/min***
- First, calculate the **renal plasma flow (RPF)** using PAH clearance: RPF = (Urine PAH conc. × Urine vol.) / Plasma PAH conc. = (30 mg/mL × 100 mL/min) / 5 mg/mL = 600 mL/min.
- Next, calculate the **glomerular filtration rate (GFR)**, which is the clearance of inulin. GFR = RPF × Filtration Fraction = 600 mL/min × 0.20 = 120 mL/min. Toxin X clearance is half of inulin clearance, so 120 mL/min / 2 = **60 mL/min**.
*1,500 ml/min*
- This value is likely obtained if an incorrect formula or conversion was made, possibly by misinterpreting the units or the relationship between GFR, RPF, and filtration fraction.
- It significantly overestimates the clearance for a substance that is cleared at half the rate of inulin.
*600 ml/min*
- This value represents the **renal plasma flow (RPF)**, calculated using the PAH clearance data.
- It does not account for the filtration fraction or the fact that toxin X clearance is half of inulin clearance (GFR).
*300 ml/min*
- This value would be obtained if the renal plasma flow (RPF) was incorrectly halved, or if an intermediate calculation was misinterpreted as the final answer.
- It does not align with the given filtration fraction and the relationship between toxin X and inulin clearance.
*120 ml/min*
- This value represents the **glomerular filtration rate (GFR)**, which is equal to the clearance of inulin (RPF × Filtration Fraction = 600 mL/min × 0.20 = 120 mL/min).
- The question states that the clearance of toxin X is **half** that of inulin, so this is an intermediate step, not the final answer.
Proximal tubule reabsorption US Medical PG Question 2: Which transport mechanism is primarily responsible for calcium reabsorption in the proximal tubule?
- A. Paracellular transport (Correct Answer)
- B. Facilitated diffusion
- C. Active transport
- D. Antiport with sodium
Proximal tubule reabsorption Explanation: ***Paracellular transport***
- In the **proximal tubule**, approximately 60-70% of filtered calcium is reabsorbed primarily through the **paracellular pathway**, driven by the electrochemical gradient and solvent drag.
- This transport occurs between cells, moving through the **tight junctions**, and is passive, following the reabsorption of water.
*Facilitated diffusion*
- While a type of passive transport, **facilitated diffusion** typically involves membrane proteins and occurs across the cell membrane, not primarily between cells in the proximal tubule for calcium.
- This mechanism is prominent for calcium reabsorption in other nephron segments like the **distal convoluted tubule** via **TRPV5/6 channels**, but not the main route in the proximal tubule.
*Active transport*
- **Active transport** of calcium, mainly via **calcium ATPase** and the **Na+/Ca2+ exchanger**, occurs across the luminal and basolateral membranes, respectively, in specific nephron segments.
- However, in the **proximal tubule**, the bulk of calcium reabsorption is passive and paracellular, not ATP-dependent active transport across cell membranes.
*Antiport with sodium*
- The **Na+/Ca2+ exchanger (NCX)** is an antiport mechanism that plays a crucial role in extruding calcium from the cell into the interstitium, particularly in the basolateral membrane of the distal tubule.
- However, it is not the primary mechanism for overall tubular reabsorption of calcium in the **proximal tubule**, where paracellular movement dominates.
Proximal tubule reabsorption US Medical PG Question 3: 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
Proximal tubule reabsorption 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.
Proximal tubule reabsorption US Medical PG Question 4: Which mechanism primarily regulates sodium reabsorption in the collecting duct?
- A. Glomerulotubular balance
- B. Atrial natriuretic peptide
- C. Antidiuretic hormone
- D. Aldosterone (Correct Answer)
Proximal tubule reabsorption Explanation: ***Aldosterone***
- **Aldosterone** is the primary hormone that stimulates **sodium reabsorption** and **potassium secretion** in the principal cells of the collecting duct.
- It acts by increasing the synthesis and activity of **ENaC channels** on the apical membrane and **Na+/K+-ATPase pumps** on the basolateral membrane.
*Glomerulotubular balance*
- **Glomerulotubular balance** refers to the mechanism by which the **proximal tubule** reabsorbs a constant fraction of the filtered load, regardless of changes in glomerular filtration rate (GFR).
- This mechanism maintains a relatively constant delivery of fluid and solutes to downstream segments but does not primarily regulate sodium in the collecting duct.
*Atrial natriuretic peptide*
- **Atrial natriuretic peptide (ANP)** primarily **inhibits sodium reabsorption** in the collecting duct, leading to **natriuresis** and **diuresis**, which is the opposite of sodium reabsorption.
- ANP is released in response to atrial stretch, indicating increased blood volume.
*Antidiuretic hormone*
- **Antidiuretic hormone (ADH)** primarily regulates **water reabsorption** in the collecting duct by increasing the insertion of **aquaporin-2 channels** into the apical membrane, making the collecting duct permeable to water.
- While ADH can indirectly affect sodium concentration by influencing water movement, it does not directly regulate sodium transport to the same extent as aldosterone.
Proximal tubule reabsorption US Medical PG Question 5: A 17-year-old boy is brought to the physician by his father because of a 7-month history of fatigue, recurrent leg cramps, and increased urinary frequency. His pulse is 94/min and blood pressure is 118/85 mm Hg. Physical examination shows dry mucous membranes. Laboratory studies show:
Serum
Na+ 130 mEq/L
K+ 2.8 mEq/L
Cl- 92 mEq/L
Mg2+ 1.1 mEq/L
Ca2+ 10.6 mg/dL
Albumin 5.2 g/dL
Urine
Ca2+ 70 mg/24 h
Cl- 375 mEq/24h (N = 110–250)
Arterial blood gas analysis on room air shows a pH of 7.55 and an HCO3- concentration of 45 mEq/L. Impaired function of which of the following structures is the most likely cause of this patient's condition?
- A. Ascending loop of Henle
- B. Collecting duct
- C. Distal convoluted tubule (Correct Answer)
- D. Descending loop of Henle
- E. Proximal convoluted tubule
Proximal tubule reabsorption Explanation: ***Distal convoluted tubule***
- The patient presents with **hypokalemia**, **metabolic alkalosis**, **hypomagnesemia**, and **hypocalciuria** (24-hour urine Ca2+ 70 mg, normal up to 250 mg), which are characteristic findings of **Gitelman syndrome**.
- **Gitelman syndrome** is caused by a loss-of-function mutation in the **thiazide-sensitive Na-Cl cotransporter (NCC)**, located in the **distal convoluted tubule**, leading to impaired reabsorption of Na+ and Cl- at this segment.
*Ascending loop of Henle*
- Impaired function of the **Na-K-2Cl cotransporter (NKCC2)** in the **thick ascending limb of the loop of Henle** causes **Bartter syndrome**.
- Bartter syndrome typically presents with **hypercalciuria**, in contrast to the hypocalciuria seen in this patient.
*Collecting duct*
- Dysfunction of the **collecting duct** can lead to various conditions, such as **renal tubular acidosis** or **diabetes insipidus**, depending on which channels or receptors are affected.
- However, the specific combination of **hypokalemia**, **metabolic alkalosis**, **hypomagnesemia**, and **hypocalciuria** points away from primary collecting duct dysfunction.
*Descending loop of Henle*
- The **descending loop of Henle** is primarily permeable to **water** and has a limited role in electrolyte reabsorption.
- Impairment here would primarily affect **urine concentration** and dilution but would not account for the specific electrolyte imbalances observed.
*Proximal convoluted tubule*
- The **proximal convoluted tubule** is responsible for reabsorbing a large fraction of filtered electrolytes, glucose, and amino acids.
- Dysfunction here (e.g., **Fanconi syndrome**) would typically present with **generalized aminoaciduria**, **glycosuria**, **phosphaturia**, and **proximal renal tubular acidosis**, which are not seen in this patient.
Proximal tubule reabsorption US Medical PG Question 6: Renal clearance of substance Y is experimentally studied. At a constant glomerular filtration rate, it is found that the amount of substance Y excreted is greater than the amount filtered. This holds true across all physiologic values on the titration curve. Substance Y is most similar to which of the following?
- A. Para-amino hippuric acid (Correct Answer)
- B. Albumin
- C. Bicarbonate
- D. Magnesium
- E. Glucose
Proximal tubule reabsorption Explanation: ***Para-amino hippuric acid***
- If the amount of a substance excreted is **greater than the amount filtered**, it indicates that the substance undergoes both **glomerular filtration** and **tubular secretion**.
- **Para-amino hippuric acid (PAH)** is a classic example of a substance that is extensively filtered and actively secreted by the renal tubules, making its clearance rate very high and a good estimate of **renal plasma flow**.
*Albumin*
- **Albumin** is a large protein that is normally **not filtered** by the glomerulus due to its size and negative charge.
- Its presence in the urine, indicating a greater amount excreted than filtered (which is normally zero), would suggest **glomerular damage**, but it does not undergo active tubular secretion.
*Bicarbonate*
- **Bicarbonate** is freely filtered at the glomerulus and is primarily **reabsorbed** in the renal tubules, particularly in the proximal tubule.
- Therefore, the amount of bicarbonate excreted is typically **much less than** the amount filtered, not greater.
*Magnesium*
- **Magnesium** is filtered by the glomeruli and undergoes complex regulation involving both **reabsorption and secretion** in various parts of the renal tubule, though reabsorption predominates.
- While magnesium balance is maintained by the kidneys, its excretion does not typically exceed filtration to the extent described for substances primarily handled by secretion.
*Glucose*
- **Glucose** is freely filtered at the glomerulus and is almost **completely reabsorbed** in the proximal tubule under normal physiological conditions.
- The amount of glucose excreted is typically zero, and only exceeds filtration when the **tubular reabsorptive capacity is saturated**, as in uncontrolled diabetes, but it is reabsorbed, not secreted.
Proximal tubule reabsorption US Medical PG Question 7: A 39-year-old woman presents to the clinic with complaints of constipation for the past 2 weeks. She reports that it has been getting increasingly difficult to pass stool to the point that she would go for 2-3 days without going to the bathroom. Prior to this, she passed stool every day without difficulty. She denies weight changes, headaches, chest pain, or abdominal pain but endorses fatigue. Her past medical history is significant for 2 episodes of kidney stones within the past 3 months. A physical examination is unremarkable. Laboratory studies are done and the results are shown below:
Serum:
Na+: 138 mEq/L
Cl-: 97 mEq/L
K+: 3.9 mEq/L
HCO3-: 24 mEq/L
BUN: 10 mg/dL
Glucose: 103 mg/dL
Creatinine: 1.1 mg/dL
Thyroid-stimulating hormone: 3.1 uU/mL
Ca2+: 12.1 mg/dL
Phosphate: 1.2 mg/dL (Normal: 2.5-4.5 mg/dL)
What is the most likely explanation for this patient’s low phosphate levels?
- A. Defective G-coupled calcium-sensing receptors in multiple tissues
- B. Increased calcium reabsorption at the distal convoluted tubule due to enhanced TRPV5 channel activity
- C. Hereditary malfunction of phosphate absorption at the small brush border
- D. Chronic renal disease caused by recurrent renal stones
- E. Inhibition of sodium-phosphate cotransporter at the proximal convoluted tubule (PCT) (Correct Answer)
Proximal tubule reabsorption Explanation: ***Inhibition of sodium-phosphate cotransporter at the proximal convoluted tubule (PCT)***
- The patient presents with **hypercalcemia (Ca2+ 12.1 mg/dL)** and **hypophosphatemia (Phosphate 1.2 mg/dL)**, along with a history of recurrent kidney stones and constipation, which are classic signs of **primary hyperparathyroidism**.
- In primary hyperparathyroidism, elevated **parathyroid hormone (PTH)** directly inhibits the **sodium-phosphate cotransporter** in the PCT, leading to decreased phosphate reabsorption and increased renal phosphate excretion.
*Defective G-coupled calcium-sensing receptors in multiple tissues*
- This describes **familial hypocalciuric hypercalcemia (FHH)**, where defective **calcium-sensing receptors (CaSRs)** in the parathyroid glands and kidneys cause a higher set point for calcium, leading to hypercalcemia.
- However, FHH typically presents with **normal to slightly elevated PTH levels** and **hypocalciuria**, whereas this patient's presentation with hypophosphatemia and recurrent kidney stones is more consistent with elevated PTH from primary hyperparathyroidism.
*Increased calcium reabsorption at the distal convoluted tubule due to enhanced TRPV5 channel activity*
- While **PTH** does increase calcium reabsorption, this occurs primarily in the **distal convoluted tubule (DCT)** via activation of **TRPV5 channels**.
- This mechanism explains the **hypercalcemia** but does not directly account for the observed **hypophosphatemia**, which is primarily due to PTH's action on phosphate excretion in the PCT.
*Hereditary malfunction of phosphate absorption at the small brush border*
- This describes conditions like **hereditary hypophosphatemic rickets**, which are characterized by isolated renal phosphate wasting and usually present earlier in life.
- This patient's acute onset of symptoms, hypercalcemia, and history of kidney stones point away from a primary hereditary defect in intestinal phosphate absorption.
*Chronic renal disease caused by recurrent renal stones*
- While recurrent kidney stones can lead to chronic kidney disease (CKD), CKD typically causes **hyperphosphatemia** due to reduced glomerular filtration of phosphate, especially in later stages.
- The patient's creatinine and BUN are within normal limits, indicating no significant chronic kidney disease that would explain the hypophosphatemia.
Proximal tubule reabsorption US Medical PG Question 8: Certain glucose transporters that are expressed predominantly on skeletal muscle cells and adipocytes are unique compared to those transporters found on other cell types within the body. Without directly affecting glucose transport in other cell types, which of the following would be most likely to selectively increase glucose uptake in skeletal muscle cells and adipocytes?
- A. Increased plasma glucose concentration
- B. It is physiologically impossible to selectively increase glucose uptake in specific cells
- C. Increased levels of circulating insulin (Correct Answer)
- D. Decreased plasma glucose concentration
- E. Decreased levels of circulating insulin
Proximal tubule reabsorption Explanation: ***Increased levels of circulating insulin***
- Insulin stimulates the translocation of **GLUT4 transporters** from intracellular vesicles to the cell membrane in **skeletal muscle** and **adipocytes**, thereby increasing glucose uptake.
- This mechanism is **selective** because other cell types (e.g., brain, liver) primarily use insulin-independent glucose transporters (e.g., GLUT1, GLUT2, GLUT3) that are constitutively active or respond to different signals.
*Increased plasma glucose concentration*
- While increased glucose concentration would drive glucose uptake in many cells, it is not **selective** for skeletal muscle and adipocytes since other cells also take up glucose.
- Insulin-independent tissues would also increase glucose uptake, making this a non-specific effect.
*It is physiologically impossible to selectively increase glucose uptake in specific cells*
- This statement is incorrect because the body has mechanisms, such as **insulin-mediated GLUT4 translocation**, that specifically regulate glucose uptake in certain cell types like skeletal muscle and adipocytes.
- This regulatory specificity is crucial for maintaining **glucose homeostasis**.
*Decreased plasma glucose concentration*
- A decrease in plasma glucose would generally **reduce** glucose uptake across all cell types, including skeletal muscle and adipocytes.
- It would not selectively increase uptake in any specific cell population.
*Decreased levels of circulating insulin*
- Decreased insulin levels would lead to **reduced** glucose uptake in insulin-sensitive tissues like skeletal muscle and adipocytes, as GLUT4 transporters would remain sequestered intracellularly.
- This would result in higher circulating glucose levels rather than increased uptake.
Proximal tubule reabsorption US Medical PG Question 9: A 30-year-old man presents to his physician for a follow-up appointment for a blood pressure of 140/90 mm Hg during his last visit. He was advised to record his blood pressure at home with an automated device twice every day. He recorded a wide range of blood pressure values in the past week, ranging from 110/70 mm Hg to 135/84 mm Hg. The medical history is unremarkable and he takes no medications. He occasionally drinks alcohol after work, but denies smoking and illicit drug use. Which of the following factors is responsible for maintaining a near-normal renal blood flow over a wide range of systemic blood pressures?
- A. Glomerular filtration
- B. Afferent arteriole (Correct Answer)
- C. Aldosterone
- D. Sympathetic nervous system
- E. Efferent arteriole
Proximal tubule reabsorption Explanation: ***Afferent arteriole***
- The **afferent arteriole** is the **primary site** of **renal autoregulation**, which maintains constant renal blood flow over a wide range of systemic blood pressures (80-180 mm Hg).
- Two key mechanisms operate here: (1) **Myogenic mechanism** - smooth muscle in the afferent arteriole constricts in response to increased stretch from elevated blood pressure, and dilates when pressure decreases; (2) **Tubuloglomerular feedback** - involves juxtaglomerular apparatus sensing changes in distal tubule NaCl delivery and adjusting afferent arteriolar tone.
- The afferent arteriole is the **initial and dominant** site where resistance changes occur to buffer pressure fluctuations before they affect glomerular capillaries.
*Glomerular filtration*
- **Glomerular filtration** is the process by which blood is filtered in the glomerulus, forming an ultrafiltrate.
- This is the **outcome** that autoregulation protects, not the mechanism itself.
- Autoregulation maintains stable GFR despite blood pressure changes.
*Aldosterone*
- **Aldosterone** is a mineralocorticoid hormone that regulates **sodium and water reabsorption** in the distal tubule and collecting duct.
- It acts over hours to days and regulates **volume status** and **chronic blood pressure control**, not acute autoregulation.
- Does not directly regulate renal blood flow in response to acute systemic blood pressure changes.
*Sympathetic nervous system*
- The **sympathetic nervous system** releases **norepinephrine**, causing **vasoconstriction** of both afferent and efferent arterioles.
- This is an **extrinsic** control mechanism that overrides autoregulation during severe stress, hemorrhage, or extreme hypotension.
- Within the **normal autoregulatory range** (80-180 mm Hg), intrinsic mechanisms (myogenic and tubuloglomerular feedback) predominate, not sympathetic control.
*Efferent arteriole*
- The **efferent arteriole** does contribute to GFR regulation, primarily through **angiotensin II-mediated constriction** which helps maintain GFR when renal perfusion pressure drops.
- However, the **primary autoregulatory adjustments** to maintain constant renal blood flow occur at the **afferent arteriole** level through the myogenic mechanism.
- The efferent arteriole plays a more significant role in maintaining GFR during hypotension rather than buffering blood flow changes across the full autoregulatory range.
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