Distal tubule and collecting duct function US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Distal tubule and collecting duct function. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Distal tubule and collecting duct function US Medical PG Question 1: A 61-year-old male is given acetazolamide to treat open-angle glaucoma. Upon diuresis, his urine is found to be highly alkaline. Which of the following accounts for the alkaline nature of this patient’s urine?
- A. Inhibition of bicarbonate reabsorption in the proximal tubule (Correct Answer)
- B. Inhibition of bicarbonate reabsorption in beta-intercalated cells
- C. Inhibition of acid secretion in alpha-intercalated cells
- D. Inhibition of chloride reabsorption in the distal convoluted tubule
- E. Inhibition of chloride reabsorption in the thick ascending loop of Henle
Distal tubule and collecting duct function Explanation: ***Inhibition of bicarbonate reabsorption in the proximal tubule***
- **Acetazolamide** is a **carbonic anhydrase inhibitor** that primarily acts on the **proximal tubule** of the kidney.
- Its action here prevents the reabsorption of **bicarbonate (HCO3-)**, leading to its increased excretion in the urine and thus making the urine alkaline.
*Inhibition of chloride reabsorption in the distal convoluted tubule*
- This effect is typically associated with **thiazide diuretics**, which inhibit the **Na-Cl cotransporter** in the distal convoluted tubule.
- While it affects electrolyte balance, it does not directly lead to the observed **alkaline urine** in the manner described.
*Inhibition of bicarbonate reabsorption in beta-intercalated cells*
- **Beta-intercalated cells** in the collecting duct secrete bicarbonate, and their inhibition would lead to **acidic urine**, not alkaline.
- They play a role in **bicarbonate secretion**, not reabsorption as seen with acetazolamide's primary action.
*Inhibition of acid secretion in alpha-intercalated cells*
- **Alpha-intercalated cells** secrete acid (H+) into the urine. Inhibiting their function would reduce acid excretion, making the urine less acidic or even alkaline.
- However, the primary mechanism of acetazolamide's effect on urine pH is through **bicarbonate wasting** in the proximal tubule, not direct inhibition of acid secretion in the collecting duct.
*Inhibition of chloride reabsorption in the thick ascending loop of Henle*
- This is the mechanism of action for **loop diuretics** like furosemide, which inhibit the **Na-K-2Cl cotransporter**.
- While loop diuretics cause significant diuresis, they do not directly lead to the pronounced **urinary alkalinization** seen with acetazolamide.
Distal tubule and collecting duct function US Medical PG Question 2: An investigator is studying patients with acute decompensated congestive heart failure. He takes measurements of a hormone released from atrial myocytes, as well as serial measurements of left atrial and left ventricular pressures. The investigator observes a positive correlation between left atrial pressures and the serum level of this hormone. Which of the following is most likely the mechanism of action of this hormone?
- A. Increases potassium excretion at the collecting ducts
- B. Constricts afferent renal arteriole
- C. Decreases sodium reabsorption at the collecting tubules (Correct Answer)
- D. Decreases reabsorption of bicarbonate in the proximal convoluted tubules
- E. Increases free water reabsorption from the distal tubules
Distal tubule and collecting duct function Explanation: ***Decreases sodium reabsorption at the collecting tubules***
- The hormone described, exhibiting a positive correlation with left atrial pressure and released from atrial myocytes, is **Atrial Natriuretic Peptide (ANP)**.
- ANP promotes **natriuresis** (sodium excretion) and **diuresis** by directly inhibiting sodium reabsorption in the collecting tubules, thereby reducing blood volume and cardiac preload.
*Increases potassium excretion at the collecting ducts*
- While ANP does promote fluid and electrolyte excretion, its primary effect is on sodium and water, not a direct increase in **potassium excretion**. **Aldosterone**, not ANP, primarily increases potassium secretion in the collecting ducts.
- This option describes a mechanism more consistent with **mineralocorticoid activity**, which is counteracted by ANP.
*Constricts afferent renal arteriole*
- ANP generally causes **vasodilation** of the afferent arteriole and constriction of the efferent arteriole, increasing glomerular filtration rate (GFR).
- **Angiotensin II** is a primary constrictor of the afferent and efferent renal arterioles, which is the opposite effect of ANP.
*Decreases reabsorption of bicarbonate in the proximal convoluted tubules*
- This mechanism is primarily involved in **acid-base balance** and is influenced by factors like parathyroid hormone or respiratory/metabolic acidosis/alkalosis.
- ANP's main action is on **sodium and water balance**, not directly on bicarbonate reabsorption.
*Increases free water reabsorption from the distal tubules*
- **Vasopressin (Antidiuretic Hormone, ADH)** is responsible for increasing free water reabsorption in the distal tubules and collecting ducts.
- ANP's action is to *increase* water excretion, working in opposition to ADH to reduce circulating fluid volume.
Distal tubule and collecting duct function US Medical PG Question 3: A 54-year-old woman comes to the physician for a follow-up examination after presenting with elevated blood pressure readings during her last two visits. After her last visit 2 months ago, she tried controlling her hypertension with weight loss before starting medical therapy, but she has since been unable to lose any weight. Her pulse is 76/min, and blood pressure is 154/90 mm Hg on the right arm and 155/93 mm Hg on the left arm. She agrees to start treatment with a thiazide diuretic. In response to this treatment, which of the following is most likely to decrease?
- A. Serum uric acid levels
- B. Urinary calcium excretion (Correct Answer)
- C. Serum glucose levels
- D. Urinary potassium excretion
- E. Urinary sodium excretion
Distal tubule and collecting duct function Explanation: ***Urinary calcium excretion***
- Thiazide diuretics work by inhibiting the **Na-Cl cotransporter** in the **distal convoluted tubule**, which leads to decreased sodium reabsorption and subsequently increased calcium reabsorption.
- This property makes thiazides useful in treating conditions like **hypercalciuria** and preventing **calcium-containing kidney stones**.
*Serum uric acid levels*
- Thiazide diuretics are known to **increase serum uric acid levels** by inhibiting its secretion in the proximal tubule.
- This can precipitate or worsen **gout attacks**, a known side effect of these medications.
*Serum glucose levels*
- Thiazide diuretics can cause **increased serum glucose levels** by impairing insulin secretion and promoting insulin resistance.
- This effect is more pronounced at higher doses and in patients with pre-existing metabolic risk factors.
*Urinary potassium excretion*
- Thiazide diuretics **increase urinary potassium excretion**, often leading to **hypokalemia**.
- This occurs because decreased sodium reabsorption in the distal convoluted tubule leads to increased sodium delivery to the collecting duct, stimulating an exchange for potassium.
*Urinary sodium excretion*
- The primary mechanism of action of thiazide diuretics is to inhibit sodium reabsorption in the distal convoluted tubule, which directly leads to an **increase in urinary sodium excretion**.
- This increased sodium excretion is what drives their diuretic and antihypertensive effects.
Distal tubule and collecting duct function US Medical PG Question 4: A 72-year-old female is brought to the emergency department after being found unresponsive in her garage with an open bottle of unmarked fluid. She is confused and is unable to answer questions on arrival. Her medical history is significant for Alzheimer disease, but her family says she has no medical comorbidities. Serum analysis of this patient's blood shows a pH of 7.28 with a high anion gap. The electrolyte that is most likely significantly decreased in this patient follows which of the following concentration curves across the proximal tubule of the kidney?
- A. Curve C
- B. Curve E
- C. Curve B
- D. Curve A
- E. Curve D (Correct Answer)
Distal tubule and collecting duct function Explanation: ***Curve D***
- The patient presents with **high anion gap metabolic acidosis**, which, in the context of an unknown fluid ingestion, is highly suggestive of **methanol** or **ethylene glycol poisoning**. These toxins are metabolized into toxic acids (**formic acid** from methanol; **glycolic acid, oxalic acid** from ethylene glycol).
- These toxic acid anions displace **bicarbonate** (HCO3-) in the blood to maintain electroneutrality, leading to a **decreased bicarbonate level**. Curve D represents bicarbonate, which is largely reabsorbed in the proximal tubule but significantly reduced in this scenario.
*Curve C*
- This curve likely represents a substance like **phosphate** or **urea**, which is partially reabsorbed and partially excreted.
- While phosphate levels can be affected in various metabolic derangements, it's not the primary electrolyte significantly decreased in **high anion gap metabolic acidosis** from toxic alcohol ingestion.
*Curve E*
- This curve typically represents a substance that is **filtered and then minimally reabsorbed** or even secreted, such as **creatinine** or **potassium** when excess is being excreted.
- **Potassium** levels can be variable in acidosis but are not typically the most significantly decreased electrolyte in this poisoning scenario.
*Curve B*
- This curve would normally represent an electrolyte that is **highly reabsorbed** in the proximal tubule, with very little remaining.
- This might represent substances like **glucose** (under normal conditions) or **amino acids**, which are not the primary electrolyte affected in this case.
*Curve A*
- This curve represents a substance that is **freely filtered** and then **neither reabsorbed nor secreted** significantly in the proximal tubule, such as **inulin**.
- This pattern does not correspond to an electrolyte whose level would be significantly decreased due to high anion gap metabolic acidosis.
Distal tubule and collecting duct function US Medical PG Question 5: 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
Distal tubule and collecting duct function 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.
Distal tubule and collecting duct function US Medical PG Question 6: 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
Distal tubule and collecting duct function 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**.
Distal tubule and collecting duct function US Medical PG Question 7: A new drug X is being tested for its effect on renal function. During the experiments, the researchers found that in patients taking substance X, the urinary concentration of sodium decreases while urine potassium concentration increase. Which of the following affects the kidneys in the same way as does substance X?
- A. Aldosterone (Correct Answer)
- B. Furosemide
- C. Spironolactone
- D. Atrial natriuretic peptide
- E. Hydrochlorothiazide
Distal tubule and collecting duct function Explanation: ***Aldosterone***
- **Aldosterone** acts on the **principal cells** of the **collecting duct** to increase sodium reabsorption and potassium secretion.
- This action leads to a decrease in urinary sodium concentration and an increase in urinary potassium concentration, matching the effects of drug X.
*Furosemide*
- **Furosemide** is a **loop diuretic** that inhibits the **Na-K-2Cl cotransporter** in the **thick ascending limb** of the loop of Henle.
- This inhibition leads to increased excretion of sodium, potassium, and water, resulting in higher urinary sodium concentration.
*Spironolactone*
- **Spironolactone** is an **aldosterone antagonist** that blocks aldosterone's effects on the collecting duct.
- This leads to increased sodium excretion and decreased potassium excretion (potassium-sparing effect), which is the opposite of drug X.
*Atrial natriuretic peptide*
- **Atrial natriuretic peptide (ANP)** is released in response to atrial stretch and causes **natriuresis** (increased sodium excretion) and **diuresis**.
- It works by dilating afferent arterioles and constricting efferent arterioles, increasing GFR, and inhibiting sodium reabsorption, thus increasing urinary sodium concentration.
*Hydrochlorothiazide*
- **Hydrochlorothiazide** is a **thiazide diuretic** that inhibits the **Na-Cl cotransporter** in the **distal convoluted tubule**.
- This leads to increased sodium and chloride excretion but typically causes potassium wasting (hypokalemia), which differs from the increased urinary potassium concentration seen with drug X.
Distal tubule and collecting duct function US Medical PG Question 8: A 55-year-old woman presents to the emergency room with severe abdominal pain for the past 24 hours. She has also noticed blood in her urine. She does not have any significant past medical history. Family history is significant for her mother having cholecystitis status post cholecystectomy at age 45. Her vital signs include: temperature 36.8°C (98.2°F), pulse 103/min, respiratory rate 15/min, blood pressure 105/85 mm Hg. Physical examination is significant for a woman continuously moving on the exam table in an attempt to get comfortable. Laboratory findings are significant for the following:
Serum electrolytes
Na 138 mEq/L N: 135–145 mEq/L
K 4.0 mEq/L N: 3.5–5.0 mEq/L
Cl 102 mEq/L N: 98–108 mEq/L
CO2 27 mEq/L N: 22–32 mEq/L
Ca 9.2 mEq/dL N: 8.4–10.2 mEq/dL
PO4 3.5 mg/dL N: 3.0–4.5 mg/dL
A 24-hour urine collection is performed and reveals a urinary calcium of 345 mg/day (ref: < 300 mg/day in men; < 250 mg/day in women). A non-contrast CT of the abdomen is performed and is shown in the exhibit. The patient’s symptoms pass within the next 12 hours with hydration and acetaminophen for pain management. She is prescribed a medication to prevent subsequent episodes. At which of the following parts of the nephron does this medication most likely work?
- A. Descending limb of the loop of Henle
- B. Distal convoluted tubule (Correct Answer)
- C. Proximal tubule
- D. Collecting ducts
- E. Thick ascending limb of the loop of Henle
Distal tubule and collecting duct function Explanation: **Distal convoluted tubule**
- The patient's symptoms (severe abdominal pain, hematuria, flank movement, and high urinary calcium) and CT findings (likely urinary calculi) point to **nephrolithiasis** (kidney stones).
- The medication prescribed to prevent subsequent episodes would most likely be a **thiazide diuretic**, which acts on the **distal convoluted tubule** to increase calcium reabsorption and decrease urinary calcium excretion.
*Descending limb of the loop of Henle*
- This segment is primarily responsible for **water reabsorption** and is permeable to water but impermeable to solutes, playing no significant role in the reabsorption of calcium that would be targeted by stone prevention medication.
- No common diuretic for nephrolithiasis prevention directly targets calcium handling in this part of the nephron.
*Proximal tubule*
- While a significant amount of calcium is reabsorbed here, medications specifically used to prevent kidney stones (like thiazides) do not primarily act on the **proximal tubule** for their calcium-sparing effects.
- The primary function of the proximal tubule is **bulk reabsorption** of filtered solutes and water.
*Collecting ducts*
- The collecting ducts are primarily involved in **fine-tuning water reabsorption** under the influence of ADH and also regulate potassium and acid-base balance.
- While some calcium reabsorption can occur here, it's not the main site of action for **thiazide diuretics** used in kidney stone prevention.
*Thick ascending limb of the loop of Henle*
- This segment is responsible for reabsorbing significant amounts of sodium, potassium, and chloride, and is the site of action for **loop diuretics**.
- **Loop diuretics** actually *increase* urinary calcium excretion and are thus contraindicated in calcium kidney stone prevention.
Distal tubule and collecting duct function US Medical PG Question 9: 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
Distal tubule and collecting duct function 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.
Distal tubule and collecting duct function US Medical PG Question 10: A 57-year-old male is found to have an elevated prostate specific antigen (PSA) level on screening labwork. PSA may be elevated in prostate cancer, benign prostatic hypertrophy (BPH), or prostatitis. Which of the following best describes the physiologic function of PSA?
- A. Regulation of transcription factors and phosphorylation of proteins
- B. Maintains corpus luteum
- C. Response to peritoneal irritation
- D. Sperm production
- E. Liquefaction of semen (Correct Answer)
Distal tubule and collecting duct function Explanation: ***Liquefaction of semen***
- Prostate-specific antigen (PSA) is a **serine protease** produced by the epithelial cells of the prostate gland.
- Its primary physiological role is to **liquefy the seminal coagulum** formed after ejaculation, allowing sperm to become motile and navigate the female reproductive tract.
*Regulation of transcription factors and phosphorylation of proteins*
- This function is characteristic of **kinases** and **phosphatases**, which are involved in intracellular signaling pathways.
- While essential for cellular function, it does not describe the specific role of PSA.
*Maintains corpus luteum*
- The maintenance of the corpus luteum is primarily the role of **luteinizing hormone (LH)** and, in pregnancy, **human chorionic gonadotropin (hCG)**.
- These hormones are involved in the female reproductive cycle, unrelated to PSA.
*Response to peritoneal irritation*
- Peritoneal irritation triggers an inflammatory response involving various immune cells and mediators, but not specifically PSA.
- PSA itself is not directly involved in the systemic or local response to peritoneal inflammation.
*Sperm production*
- **Sperm production (spermatogenesis)** occurs in the seminiferous tubules of the testes under the influence of hormones like FSH and testosterone.
- While semen is the vehicle for sperm, PSA's role is in the post-ejaculatory processing of semen, not in the initial production of sperm.
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