Carbonic Anhydrase Inhibitors — MCQs
What is the mechanism of action of acetazolamide in the treatment of glaucoma?
Which drug is given to prevent acute mountain sickness?
Side effects of oral carbonic anhydrase inhibitors include?
Which of the following statements about the PRIMARY mechanism of action of acetazolamide is correct?
Carbonic anhydrase inhibitor should not be given in:
Which of the following is a carbonic anhydrase inhibitor?
In which segment of the nephron does ethacrynic acid exert its diuretic action?
All of the following diuretics increase K+ excretion EXCEPT:
What is the primary effect of acetazolamide when used at high altitudes?
Renal stones are seen as a complication by using the following drug:
Carbonic Anhydrase Inhibitors Indian Medical PG Practice Questions and MCQs
Question 1: What is the mechanism of action of acetazolamide in the treatment of glaucoma?
- A. Increases aqueous humor production
- B. Increases intraocular pressure
- C. Decreases aqueous outflow
- D. Decreases aqueous production (Correct Answer)
Explanation: ***Decreases aqueous production*** - **Acetazolamide** is a **carbonic anhydrase inhibitor** that acts on the **ciliary body** to reduce the formation of **bicarbonate ions** [2]. - This reduction in bicarbonate ions subsequently decreases the amount of fluid and solutes entering the **posterior chamber**, thereby lowering **aqueous humor production** and **intraocular pressure (IOP)** [2]. *Increases aqueous humor production* - This is incorrect; acetazolamide's therapeutic effect in glaucoma is specifically to **decrease** aqueous humor production [2]. - Increased aqueous humor production would lead to an **increase** in intraocular pressure, worsening glaucoma [1]. *Increases intraocular pressure* - This is incorrect; the primary goal of acetazolamide in glaucoma treatment is to **lower intraocular pressure**, not increase it [2]. - An increase in IOP is detrimental in glaucoma, leading to optic nerve damage [1]. *Decreases aqueous outflow* - This is incorrect; acetazolamide does not primarily affect the outflow pathways (e.g., **trabecular meshwork** or **uveoscleral outflow**) [1]. - A decrease in aqueous outflow would lead to an **increase in IOP**, which is contrary to the drug's intended action [1].
Question 2: Which drug is given to prevent acute mountain sickness?
- A. Acetazolamide (Correct Answer)
- B. Diltiazem
- C. Digoxin
- D. Dexamethasone
Explanation: ***Acetazolamide*** - This drug is a **carbonic anhydrase inhibitor** that acidifies the blood and causes compensatory hyperventilation, increasing oxygenation. - It is the **first-line prophylactic agent** for acute mountain sickness (AMS) and is best started 24-48 hours before ascent. - Most effective and widely recommended for AMS prevention. *Digoxin* - This is a **cardiac glycoside** used to treat heart failure and irregular heartbeats. - Its mechanism of action is unrelated to the physiological changes that cause acute mountain sickness. *Diltiazem* - This is a **calcium channel blocker** primarily used for hypertension, angina, and certain arrhythmias. - It has no known role in the prevention or treatment of acute mountain sickness. *Dexamethasone* - While **dexamethasone** can be used for AMS prophylaxis, it is typically reserved as an **alternative agent** when acetazolamide is contraindicated or not tolerated. - It is more commonly used for **treatment** of severe altitude illness including **High Altitude Cerebral Edema (HACE)** and **High Altitude Pulmonary Edema (HAPE)**. - **Acetazolamide remains the preferred first-line prophylactic agent** due to its mechanism of action that directly addresses the underlying pathophysiology of AMS.
Question 3: Side effects of oral carbonic anhydrase inhibitors include?
- A. Paresthesia (Correct Answer)
- B. Hyperkalemia
- C. Insomnia
- D. Weight gain
Explanation: ***Paresthesia*** - **Paresthesia** (tingling sensation, especially in fingers, toes, and perioral region) is one of the **most common side effects** of carbonic anhydrase inhibitors like acetazolamide - Occurs due to **systemic metabolic acidosis** and altered neuronal excitability from pH shifts and electrolyte changes - This symptom is so common that patients should be counseled about it before starting therapy *Hyperkalemia* - Carbonic anhydrase inhibitors cause **hypokalemia (potassium wasting)**, NOT hyperkalemia - Mechanism: Increased sodium delivery to distal tubule → increased Na-K exchange → urinary potassium loss - Although metabolic acidosis can shift potassium out of cells, the **diuretic effect predominates**, leading to net potassium loss *Insomnia* - **Not a recognized side effect** of carbonic anhydrase inhibitors - Patients may actually experience **fatigue or drowsiness** rather than insomnia - No direct pharmacological mechanism links carbonic anhydrase inhibition to sleep disturbances *Weight gain* - Carbonic anhydrase inhibitors have a **diuretic effect** causing fluid and sodium loss - Patients typically experience **weight loss or stable weight**, not weight gain - Weight gain is associated with fluid-retaining medications, which is the opposite effect of diuretics
Question 4: Which of the following statements about the PRIMARY mechanism of action of acetazolamide is correct?
- A. It causes metabolic alkalosis
- B. It decreases potassium excretion
- C. Has structural resemblance to penicillins
- D. Reversible inhibitor of carbonic anhydrase (Correct Answer)
Explanation: ***Reversible inhibitor of carbonic anhydrase*** - Acetazolamide is a **carbonic anhydrase inhibitor** that primarily acts by reducing the reabsorption of bicarbonate in the proximal tubule of the kidney. - This inhibition leads to increased urinary excretion of **bicarbonate, sodium, and water**, ultimately reducing intraocular pressure, intracranial pressure, and systemic fluid retention. *It decreases potassium excretion* - Acetazolamide actually **increases potassium excretion** indirectly by increasing the delivery of sodium and bicarbonate to the collecting duct, which enhances potassium secretion. - This can lead to **hypokalemia** with prolonged use. *It causes metabolic alkalosis* - Acetazolamide causes **metabolic acidosis**, not alkalosis, because it inhibits the reabsorption of bicarbonate in the renal tubules, leading to increased bicarbonate excretion and a decrease in serum bicarbonate levels. - This effect makes it useful in treating metabolic alkalosis, but it does not cause it. *Has structural resemblance to penicillins* - Acetazolamide is a **sulfonamide derivative** and does not have a structural resemblance to penicillins. - Its mechanism of action is completely distinct from that of antibiotics in the penicillin class.
Question 5: Carbonic anhydrase inhibitor should not be given in:
- A. Epilepsy
- B. High altitude sickness
- C. Sulfonamide hypersensitivity (Correct Answer)
- D. Glaucoma
Explanation: ***Sulfonamide hypersensitivity*** - Carbonic anhydrase inhibitors (CAIs) are **sulfonamide derivatives**, so they are absolutely contraindicated in patients with a history of **sulfonamide allergy**. - Administration to such patients can lead to severe **hypersensitivity reactions**, including rash, fever, and even anaphylaxis. *Epilepsy* - **Acetazolamide**, a carbonic anhydrase inhibitor, can be used as an **adjunct therapy for certain types of epilepsy**, such as absence seizures. - It works by reducing neuronal excitability through its effects on pH, thus it is not contraindicated but rather sometimes indicated. *High altitude sickness* - Carbonic anhydrase inhibitors like **acetazolamide** are commonly used to **prevent and treat high altitude cerebral and pulmonary edema** by inducing metabolic acidosis and stimulating respiration. - This is a recognized therapeutic indication, not a contraindication. *Glaucoma* - CAIs are a **primary treatment for glaucoma** (both open-angle and angle-closure) because they reduce the production of aqueous humor, thereby lowering intraocular pressure. - They are used both systemically and topically for this purpose, making it an indication, not a contraindication.
Question 6: Which of the following is a carbonic anhydrase inhibitor?
- A. Hydrochlorothiazide
- B. Mannitol
- C. Furosemide
- D. Acetazolamide (Correct Answer)
Explanation: ***Acetazolamide*** - Acetazolamide is a classic example of a **carbonic anhydrase inhibitor**, primarily used as a diuretic and for managing **glaucoma** and **altitude sickness**. - It works by inhibiting the enzyme **carbonic anhydrase** in the **proximal renal tubule**, reducing bicarbonate reabsorption and thus promoting diuresis. *Hydrochlorothiazide* - Hydrochlorothiazide is a **thiazide diuretic** that acts on the **distal convoluted tubule** to inhibit the reabsorption of sodium and chloride. - It is not a carbonic anhydrase inhibitor. *Mannitol* - Mannitol is an **osmotic diuretic** that works in the **proximal tubule** and **descending limb of Henle's loop** by creating an osmotic gradient. - It is not a carbonic anhydrase inhibitor and functions by drawing water into the renal tubule, leading to increased urine output. *Furosemide* - Furosemide is a **loop diuretic** that acts on the **thick ascending limb of the loop of Henle** by inhibiting the Na+-K+-2Cl- cotransporter. - It is one of the most potent diuretics but does not inhibit carbonic anhydrase.
Question 7: In which segment of the nephron does ethacrynic acid exert its diuretic action?
- A. Proximal convoluted tubule
- B. Collecting duct
- C. Distal convoluted tubule
- D. Thick ascending limb of loop of Henle (Correct Answer)
Explanation: ***Thick ascending limb of loop of Henle*** - Ethacrynic acid is a **loop diuretic** that acts by inhibiting the **Na+-K+-2Cl- cotransporter** (NKCC2) in the luminal membrane of the thick ascending limb. - This inhibition prevents the reabsorption of ions, leading to increased excretion of water, sodium, chloride, and potassium. *Proximal convoluted tubule* - The proximal convoluted tubule is the primary site of reabsorption of most filtered substances, but loop diuretics like ethacrynic acid do not primarily act here. - Carbonic anhydrase inhibitors and SGLT2 inhibitors are examples of diuretics that exert their effects in this segment. *Collecting duct* - The collecting duct is the site where aldosterone antagonists (e.g., spironolactone) and epithelial sodium channel (ENaC) inhibitors (e.g., amiloride, triamterene) exert their diuretic effects. - Its primary role involves fine-tuning water reabsorption under the influence of ADH and regulating potassium excretion. *Distal convoluted tubule* - Thiazide diuretics primarily act in the distal convoluted tubule by inhibiting the **Na+-Cl- cotransporter** (NCC). - This segment is responsible for further diluting the urine and reabsorbing a small percentage of filtered sodium and chloride.
Question 8: All of the following diuretics increase K+ excretion EXCEPT:
- A. Acetazolamide
- B. Triamterene (Correct Answer)
- C. Thiazide
- D. Furosemide
Explanation: ***Triamterene*** - **Triamterene** is a **potassium-sparing diuretic** that blocks epithelial sodium channels (ENaC) in the collecting duct, thereby reducing sodium reabsorption and potassium secretion. - Unlike most other diuretics, it causes **decreased K+ excretion** and can lead to hyperkalemia. *Acetazolamide* - **Acetazolamide** is a **carbonic anhydrase inhibitor** that acts in the proximal tubule, inhibiting bicarbonate reabsorption. - This leads to increased delivery of sodium and bicarbonate to the collecting duct, which enhances **potassium secretion** and increases K+ excretion. *Thiazide* - **Thiazide diuretics** (e.g., hydrochlorothiazide) act by inhibiting the Na+/Cl- cotransporter in the **distal convoluted tubule**. - This increases the delivery of sodium to the collecting duct, which stimulates the exchange of sodium for **potassium**, leading to increased K+ excretion and hypokalemia. *Furosemide* - **Furosemide** is a **loop diuretic** that inhibits the Na+/K+/2Cl- cotransporter in the **thick ascending limb of the loop of Henle**. - This prevents the reabsorption of these ions, leading to increased delivery of sodium to the collecting duct, which promotes **potassium secretion** and increased K+ excretion.
Question 9: What is the primary effect of acetazolamide when used at high altitudes?
- A. Elevate pH
- B. Decrease ventilation
- C. Reduce HCO3- (Correct Answer)
- D. Increase CO2
Explanation: ***Reduce HCO3-*** - Acetazolamide is a **carbonic anhydrase inhibitor**, which primarily acts on the kidneys to block **bicarbonate reabsorption**. - By reducing **bicarbonate levels (HCO3-)**, it induces a **metabolic acidosis**, which in turn stimulates **respiratory drive** and increases ventilation, thereby counteracting the effects of altitude sickness. *Elevate pH* - Acetazolamide causes a **metabolic acidosis**, leading to a *decrease* in blood pH, not an elevation. - The reduced pH is the stimulus for **increased ventilation**, which helps to normalize blood gases at altitude. *Decrease ventilation* - The **metabolic acidosis** induced by acetazolamide actually *stimulates* peripheral chemoreceptors, leading to an *increase* in ventilation. - Increased ventilation helps to **excrete CO2**, thereby improving oxygenation at high altitudes. *Increase CO2* - Increased ventilation (due to the **metabolic acidosis** induced by acetazolamide) leads to a *decrease* in partial pressure of **carbon dioxide (PCO2)**, not an increase. - This reduction in **PCO2** helps to improve the body's acid-base balance and oxygenation.
Question 10: Renal stones are seen as a complication by using the following drug:
- A. Zonisamide (Correct Answer)
- B. Oxcarbazepine
- C. Phenytoin
- D. Tiagabine
Explanation: ***Zonisamide*** - **Zonisamide** is a sulfonamide derivative that can inhibit **carbonic anhydrase**, leading to metabolic acidosis and increased urinary calcium excretion, which promotes the formation of **kidney stones**. - Patients on zonisamide should be monitored for **renal stone formation** and advised to maintain adequate hydration. *Oxcarbazepine* - **Oxcarbazepine** is an antiepileptic drug known for causing **hyponatremia** and, less commonly, dermatological reactions such as rash. - It is not typically associated with a significant risk of **renal stone formation**. *Phenytoin* - **Phenytoin** is an older antiepileptic drug commonly associated with side effects such as **gingival hyperplasia**, hirsutism, and folate deficiency. - While it has various side effects, **nephrolithiasis** (kidney stones) is not a common or recognized complication. *Tiagabine* - **Tiagabine** is an antiepileptic drug that works by inhibiting GABA reuptake. Its main side effects include dizziness, weakness, and somnolence. - There is no significant evidence to suggest that **tiagabine** causes **renal stone formation**.
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