Renal Regulation of Acid-Base Balance — MCQs
All of the following substances have decreased concentration on the luminal side of the proximal convoluted tubule except:
A patient in renal failure exhibits metabolic acidosis. What compensatory mechanism is most likely activated?
A patient has hyperaldosteronism. Which lab finding is expected?
What is the primary mechanism for maintaining acid-base balance during prolonged vomiting?
The interpretation of the following ABG value is: pH = 7.5, pCO2 = 50 mm Hg, HCO3 = 30 mEq/L
A person with type 1 diabetes ran out of her prescription insulin and has not been able to inject insulin for the past 3 days. The patient is hyperventilating to compensate for her metabolic acidosis. Which of the following reactions explains this respiratory compensation for metabolic acidosis?
Carbonic anhydrase activity is found in all of the following except?
Assertion: In a patient with chronic kidney disease (CKD) and metabolic acidosis, sodium bicarbonate should be initiated to correct acidosis. Reason: Sodium bicarbonate therapy reduces the progression of kidney disease by decreasing tubular injury and slowing fibrosis.
In a comatose patient with a blood glucose level of 750 mg/dL, which test is most important to perform in addition to serum potassium?
What is the primary enzyme responsible for the conversion of carbon dioxide to bicarbonate in erythrocytes?
Renal Regulation of Acid-Base Balance Indian Medical PG Practice Questions and MCQs
Question 1: All of the following substances have decreased concentration on the luminal side of the proximal convoluted tubule except:
- A. Glucose
- B. Amino acids
- C. Bicarbonate
- D. Chloride (Correct Answer)
Explanation: ***Chloride*** - As **water and other solutes** are reabsorbed from the proximal tubule, the concentration of **chloride** actually increases in the remaining luminal fluid due to continued water reabsorption. - This increased luminal **chloride concentration** then drives passive reabsorption of chloride later in the tubule. *Glucose* - **Glucose** is almost completely reabsorbed from the tubular lumen by **secondary active transport** in the early part of the proximal tubule. - Therefore, its concentration in the remaining luminal fluid rapidly decreases. *Amino acids* - Similar to glucose, **amino acids** are extensively reabsorbed by **secondary active transport** mechanisms in the proximal tubule. - Consequently, their luminal concentration significantly decreases. *Bicarbonate* - Most **bicarbonate** is reabsorbed in the proximal tubule through a process involving **carbonic anhydrase**, converting it to CO2 and water, which then diffuse into the cell. - This efficient reabsorption results in a substantial decrease in luminal bicarbonate concentration.
Question 2: A patient in renal failure exhibits metabolic acidosis. What compensatory mechanism is most likely activated?
- A. Hyperventilation (Correct Answer)
- B. Hypoventilation
- C. Increased renal HCO3- reabsorption
- D. Increased K+ excretion
Explanation: ***Hyperventilation*** - In metabolic acidosis, the body responds by increasing **respiratory rate and depth** to exhale more CO2, thereby reducing carbonic acid levels and raising pH. - This is a rapid compensatory mechanism to counteract the drop in blood pH caused by the accumulation of non-volatile acids or loss of bicarbonate. - In renal failure, this becomes the **primary compensatory mechanism** since renal compensation is impaired. *Hypoventilation* - **Hypoventilation** leads to CO2 retention, which would worsen metabolic acidosis by increasing carbonic acid and lowering pH further. - This mechanism is characteristic of primary respiratory acidosis, not a compensatory response to metabolic acidosis. *Increased renal HCO3- reabsorption* - While increased **renal bicarbonate reabsorption** and hydrogen ion excretion are fundamental renal compensatory mechanisms for metabolic acidosis, these are impaired in a patient with **renal failure**. - The kidneys are failing to perform this crucial function, which is the underlying cause of the metabolic acidosis in this scenario. - This is why respiratory compensation becomes the only available mechanism. *Increased K+ excretion* - **Increased K+ excretion** (or retention) is primarily a response to changes in potassium balance, though acid-base disturbances can influence it. - It is not a direct or primary compensatory mechanism for metabolic acidosis, although some renal tubular processes related to acid-base balance can affect potassium handling.
Question 3: A patient has hyperaldosteronism. Which lab finding is expected?
- A. Metabolic acidosis
- B. Hyperkalemia
- C. Hypokalemia (Correct Answer)
- D. Hyponatremia
Explanation: ***Hypokalemia*** - **Aldosterone** increases the excretion of **potassium** in the kidneys, leading to decreased serum potassium levels [1]. - This effect is mediated by aldosterone's action on the principal cells of the collecting duct, promoting potassium secretion into the urine [1]. *Metabolic acidosis* - **Hyperaldosteronism** typically causes **metabolic alkalosis** due to increased hydrogen ion excretion by the kidneys [1]. - Aldosterone promotes the reabsorption of sodium and water, and the excretion of potassium and hydrogen ions, leading to alkalosis [2]. *Hyperkalemia* - **Aldosterone's primary role** is to promote **potassium excretion** in the kidneys [1]. - Therefore, **excessive aldosterone** production would lead to **hypokalemia**, not hyperkalemia. *Hyponatremia* - **Aldosterone** promotes **sodium reabsorption** in the kidneys, which usually leads to normal or even slightly elevated serum sodium levels [1]. - **Hyponatremia** would be an unexpected finding in hyperaldosteronism [3].
Question 4: What is the primary mechanism for maintaining acid-base balance during prolonged vomiting?
- A. Increased chloride reabsorption
- B. Increased potassium excretion
- C. Increased bicarbonate excretion (Correct Answer)
- D. Decreased hydrogen secretion
Explanation: ***Increased bicarbonate excretion*** - Prolonged vomiting leads to the loss of **gastric acid (HCl)**, causing **metabolic alkalosis**. The kidneys compensate by increasing the excretion of **bicarbonate (HCO3-)** to restore acid-base balance. - This renal compensation is the primary mechanism to eliminate the excess alkali from the body. *Increased chloride reabsorption* - In **metabolic alkalosis** due to vomiting, the body tends to reabsorb less chloride, not more, in an attempt to excrete bicarbonate. - **Chloride depletion** can actually hinder bicarbonate excretion by promoting sodium reabsorption with bicarbonate. *Increased potassium excretion* - **Hypokalemia** can occur with prolonged vomiting due to increased aldosterone activity and direct renal loss associated with metabolic alkalosis. - However, increased potassium excretion itself is not the primary mechanism for correcting the acid-base disorder; rather, it is a consequence or a contributing factor to the imbalance. *Decreased hydrogen secretion* - In response to alkalosis, the kidneys would typically decrease, not increase, **hydrogen ion (H+) secretion** in an effort to retain H+ and normalize pH. - Decreased H+ secretion is a compensatory mechanism, but the direct excretion of bicarbonate is more crucial for correcting the metabolic alkalosis.
Question 5: The interpretation of the following ABG value is: pH = 7.5, pCO2 = 50 mm Hg, HCO3 = 30 mEq/L
- A. Respiratory acidosis
- B. Metabolic acidosis
- C. Metabolic alkalosis (Correct Answer)
- D. Normal acid-base balance
Explanation: ***Metabolic alkalosis (partially compensated)*** - The **pH of 7.5** indicates **alkalosis**, and the elevated **bicarbonate (HCO3) of 30 mEq/L** is the primary driver of this high pH. - The elevated **pCO2 of 50 mm Hg** represents **partial respiratory compensation**, where the body retains CO2 to lower the pH toward normal. - Since the pH remains elevated (not normalized to 7.35-7.45), this is **partially compensated** rather than fully compensated. *Respiratory acidosis* - This would be characterized by a **low pH** and an **elevated pCO2**, which is not seen here as the pH is high. - Although pCO2 is elevated, the **high pH** and **high bicarbonate** rule out primary respiratory acidosis. *Metabolic acidosis* - This would present with a **low pH** and a **low bicarbonate** concentration. - The given values show a **high pH** and **high bicarbonate**, which is the opposite of metabolic acidosis. *Normal acid-base balance* - A normal acid-base balance would have a **pH between 7.35-7.45**, a **pCO2 between 35-45 mm Hg**, and an **HCO3 between 22-26 mEq/L**. - All three values are outside of their normal ranges, indicating an acid-base disturbance.
Question 6: A person with type 1 diabetes ran out of her prescription insulin and has not been able to inject insulin for the past 3 days. The patient is hyperventilating to compensate for her metabolic acidosis. Which of the following reactions explains this respiratory compensation for metabolic acidosis?
- A. H2O ⇌ H+ + OH-
- B. H+ + NH3 ⇌ NH4+
- C. CH3CHOHCH2COOH ⇌ CH3CHOHCH2COO- + H+
- D. CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3- (Correct Answer)
Explanation: ***CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-*** - This reaction represents the **bicarbonate buffer system**, which is central to maintaining **pH balance** in the body. - In response to **metabolic acidosis**, the body hyperventilates to **decrease CO2** levels, shifting the equilibrium to the left and reducing H+ which compensates for the increased acidity. *H2O ⇌ H+ + OH-* - This reaction describes the **autoionization of water**, which is fundamental but does not directly explain the body's respiratory compensation mechanism for metabolic acidosis. - While it shows the presence of H+ ions, it doesn't illustrate how the respiratory system manipulates CO2 to influence pH. *H+ + NH3 ⇌ NH4+* - This reaction represents the **ammonia buffer system** primarily active in the **kidneys** for acid excretion. - It plays a role in renal compensation for pH imbalances, but it is not the mechanism for respiratory compensation. *CH3CHOHCH2COOH ⇌ CH3CHOHCH2COO- + H+* - This represents the **dissociation of beta-hydroxybutyric acid**, a **ketone body** produced in diabetic ketoacidosis (DKA). - While DKA is the cause of the metabolic acidosis in this patient, this specific reaction describes the *production* of H+ ions, not the *respiratory compensatory mechanism* to address it.
Question 7: Carbonic anhydrase activity is found in all of the following except?
- A. Brain
- B. Kidney
- C. RBC
- D. Plasma (Correct Answer)
Explanation: ***Plasma*** - **Carbonic anhydrase** is an intracellular enzyme that catalyzes the rapid interconversion of carbon dioxide and water to carbonic acid, **bicarbonate**, and protons. - It is notably **absent in plasma** in healthy individuals, as it is primarily found within cells where its function is crucial for pH regulation and CO2 transport. *Brain* - Carbonic anhydrase is found in various brain cells, including **neurons**, **oligodendrocytes**, and **astrocytes**. - It plays a vital role in pH regulation, fluid balance, and the production of cerebrospinal fluid (CSF) within the **central nervous system**. *Kidney* - The kidney is rich in carbonic anhydrase, particularly in the **proximal tubules** and collecting ducts. - It is critical for **bicarbonate reabsorption** and proton excretion, essential processes for maintaining acid-base balance. *RBC* - **Red blood cells (RBCs)** contain a high concentration of carbonic anhydrase (specifically CA-I and CA-II isoforms). - This enzyme facilitates the rapid conversion of CO2 to bicarbonate for transport to the lungs and the reverse reaction for **CO2 exhalation**.
Question 8: Assertion: In a patient with chronic kidney disease (CKD) and metabolic acidosis, sodium bicarbonate should be initiated to correct acidosis. Reason: Sodium bicarbonate therapy reduces the progression of kidney disease by decreasing tubular injury and slowing fibrosis.
- A. Assertion is false, but Reason is true
- B. Both Assertion and Reason are true, and Reason is the correct explanation of Assertion
- C. Assertion is true, but Reason is false
- D. Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion (Correct Answer)
Explanation: The **Assertion** is true: **KDIGO guidelines** recommend sodium bicarbonate therapy for CKD patients when serum bicarbonate falls below **22 mEq/L** to correct metabolic acidosis [2]. - The **Reason** is also true: studies demonstrate that bicarbonate therapy has **nephroprotective effects**, reducing CKD progression through decreased **tubular injury** and **interstitial fibrosis**. However, this describes a secondary benefit rather than the primary indication for initiating therapy. *Both Assertion and Reason are true, and Reason is the correct explanation of Assertion* - While both statements are medically accurate, the Reason does not explain the primary indication for bicarbonate initiation in CKD patients. - The main purpose is **acid-base correction** and prevention of acidosis complications like **bone disease**, **muscle wasting**, and **cardiovascular effects**, not primarily nephroprotection [1], [2]. *Assertion is false, but Reason is true* - The Assertion is medically correct: sodium bicarbonate is **standard therapy** for metabolic acidosis in CKD according to nephrology guidelines. - CKD patients develop acidosis due to impaired **renal acid excretion** and reduced **bicarbonate regeneration**, making correction clinically necessary [2]. *Assertion is true, but Reason is false* - The Reason is actually supported by **clinical evidence**: randomized controlled trials show bicarbonate therapy slows CKD progression. - Mechanisms include reduced **complement activation**, decreased **endothelin production**, and preservation of **residual kidney function**.
Question 9: In a comatose patient with a blood glucose level of 750 mg/dL, which test is most important to perform in addition to serum potassium?
- A. Serum creatinine
- B. Serum sodium
- C. Serum ketones
- D. Arterial blood gases (Correct Answer)
Explanation: ***Arterial blood gases*** - In a comatose patient with severe hyperglycemia (750 mg/dL), **arterial blood gases (ABGs)** are crucial to assess for **acidosis**, which could indicate **diabetic ketoacidosis (DKA)** or **hyperosmolar hyperglycemic state (HHS)** with lactic acidosis [1], [4]. - The **pH**, **bicarbonate (HCO3-)**, and **pCO2** levels from ABGs help determine the severity and type of metabolic derangement, guiding immediate treatment, especially for potential **cerebral edema** [3], [4]. *Serum creatinine* - While important for assessing **kidney function** in hyperosmolar states, it does not directly evaluate the immediate acid-base status that is critical for neurologic function in a comatose patient. - Renal insufficiency can exacerbate electrolyte imbalances and fluid overload but is secondary to the immediate need for acid-base assessment. *Serum sodium* - **Serum sodium** is important for calculating **effective serum osmolality**, which is elevated in both DKA and HHS, contributing to mental status changes [2]. - However, while important, it does not provide information about the **acid-base balance**, which is a more critical determinant of immediate neurologic stability and treatment in deep coma. *Serum ketones* - **Serum ketones** are essential for distinguishing between **DKA** (high ketones) and **HHS** (low or absent ketones) [4]. - While vital for diagnosis, ketones alone do not give the full picture of **acid-base status** (pH, bicarbonate) which is directly assessed by ABGs and more immediately actionable in managing a severely ill, comatose patient [1].
Question 10: What is the primary enzyme responsible for the conversion of carbon dioxide to bicarbonate in erythrocytes?
- A. The high solubility of CO2 in water
- B. The role of hemoglobin in CO2 transport
- C. The conversion of carbon dioxide to carbonic acid
- D. The action of carbonic anhydrase in erythrocytes (Correct Answer)
Explanation: ***The action of carbonic anhydrase in erythrocytes*** - **Carbonic anhydrase** is an enzyme found in high concentrations within **red blood cells (erythrocytes)**, catalyzing the rapid interconversion of carbon dioxide and water to **carbonic acid**. - This enzyme is crucial for the efficient transport of carbon dioxide from the tissues to the lungs, as carbonic acid quickly dissociates into **bicarbonate ions**, which are easily transported in the plasma. *The high solubility of CO2 in water* - While **CO2** does have some solubility in water, this process is too slow on its own to account for the rapid and efficient transport of the large amounts of metabolic CO2 produced by the body. - The direct dissolution of CO2 in plasma accounts for only a small fraction of its total transport. *The role of hemoglobin in CO2 transport* - **Hemoglobin** does play a role in CO2 transport by forming **carbaminohemoglobin**, binding to the amino groups on the globin chains. - However, this mechanism represents only about 20-30% of CO2 transport and does not involve the conversion to **bicarbonate**. *The conversion of carbon dioxide to carbonic acid* - The conversion of CO2 to **carbonic acid (H2CO3)** is indeed an intermediate step in bicarbonate formation. - However, this reaction is very slow in the absence of an enzyme and does not address the primary catalyst responsible for this rapid conversion.
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