Drug Classification · Questions 1–6
Identify the pharmacological class or categorical label for each drug or receptor. Vocabulary preparation is sufficient to answer every question in this section.
Question 1
Which of the following drugs is classified as a loop diuretic?
Correct Answer
B — Furosemide
Rationale
Furosemide is a loop diuretic that acts at the thick ascending limb of the loop of Henle. Hydrochlorothiazide is a thiazide diuretic. Spironolactone is a mineralocorticoid receptor antagonist (potassium-sparing diuretic). Acetazolamide is a carbonic anhydrase inhibitor.
Question 2
Which of the following drugs is classified as a thiazide-type diuretic?
Correct Answer
A — Chlorthalidone
Rationale
Chlorthalidone is a thiazide-type diuretic with a half-life of 40 to 60 hours, far exceeding that of conventional thiazides. Furosemide and bumetanide are loop diuretics. Spironolactone is a mineralocorticoid receptor antagonist.
Question 3
Which of the following loop diuretics is the only member of the class that is not a sulfonamide derivative?
Correct Answer
D — Ethacrynic acid
Rationale
Ethacrynic acid is the only loop diuretic that is not a sulfonamide derivative, which distinguishes it as the agent of choice when true sulfonamide hypersensitivity precludes the other loop diuretics. Furosemide, torsemide, and bumetanide are all sulfonamide derivatives.
Question 4
Which of the following drug pairs are both classified as loop diuretics?
Correct Answer
C — Furosemide and torsemide
Rationale
Furosemide and torsemide are both loop diuretics acting at the Na-K-2Cl cotransporter in the thick ascending limb. Hydrochlorothiazide and chlorthalidone are both thiazide or thiazide-type diuretics. Spironolactone and amiloride are both potassium-sparing diuretics. Acetazolamide is a carbonic anhydrase inhibitor and metolazone is a thiazide-type agent.
Question 5
Which of the following drugs is NOT classified as a loop diuretic?
Correct Answer
A — Hydrochlorothiazide
Rationale
Hydrochlorothiazide is a thiazide diuretic that acts at the distal convoluted tubule, not a loop diuretic. Furosemide, ethacrynic acid, and bumetanide are all loop diuretics that act at the thick ascending limb of the loop of Henle.
Question 6
Which of the following drugs is classified as a thiazide-type diuretic known to retain diuretic efficacy at low glomerular filtration rates?
Correct Answer
B — Metolazone
Rationale
Metolazone is classified as a thiazide-type diuretic that retains efficacy even at glomerular filtration rates below 30 milliliters per minute, where conventional thiazides lose effectiveness. Furosemide and torsemide are loop diuretics. Hydrochlorothiazide is a thiazide diuretic that loses efficacy in significant renal impairment.
Core Pharmacology · Questions 7–14
Apply your understanding of drug mechanisms, pharmacokinetics, and adverse effects. Each question requires one reasoning step.
Question 7
A patient with severe heart failure is started on intravenous furosemide for acute volume overload. Which of the following transporters is the primary target of furosemide in the kidney?
Correct Answer
D — Sodium-potassium-2-chloride cotransporter in the thick ascending limb
Rationale
Furosemide blocks the sodium-potassium-2-chloride cotransporter isoform 2 (Na-K-2Cl cotransporter) at its chloride-binding site in the thick ascending limb of the loop of Henle. This is the primary target responsible for the potent natriuresis produced by loop diuretics. The epithelial sodium channel is targeted by potassium-sparing diuretics. The sodium-chloride cotransporter is targeted by thiazides. The sodium-hydrogen exchanger is targeted by carbonic anhydrase inhibitors.
Question 8
A patient with symptomatic hypercalcemia is treated with intravenous normal saline followed by a loop diuretic. Which of the following best explains why loop diuretics increase urinary calcium excretion?
Correct Answer
B — Abolition of the lumen-positive electrical potential that drives paracellular calcium reabsorption
Rationale
Loop diuretics block the Na-K-2Cl cotransporter, abolishing the lumen-positive electrical potential normally generated in the thick ascending limb. This potential drives paracellular reabsorption of calcium and magnesium. When it is eliminated, calcium (and magnesium) cannot be reabsorbed paracellularly and are lost in the urine, producing calciuresis. This mechanism underlies the clinical use of loop diuretics to treat hypercalcemia.
Question 9
A patient with recurrent calcium kidney stones is prescribed a thiazide diuretic to reduce stone recurrence. Which of the following best explains how thiazide diuretics reduce urinary calcium excretion?
Correct Answer
A — Reduced intracellular sodium in the distal convoluted tubule cell enhances basolateral sodium-calcium exchange, increasing luminal calcium entry via TRPV5
Rationale
By blocking the sodium-chloride cotransporter, thiazides reduce intracellular sodium in the distal convoluted tubule cell. This enhances basolateral sodium-calcium exchange, creating a favorable gradient that draws calcium from the tubular lumen into the cell through the apical calcium channel TRPV5. The net result is calcium retention in the body and reduced urinary calcium excretion — the opposite of loop diuretics. This mechanism underlies the use of thiazides in calcium nephrolithiasis prevention.
Question 10
A patient with advanced chronic kidney disease receives standard doses of furosemide but shows a minimal diuretic response. Which of the following best explains why loop diuretics require dose escalation in chronic kidney disease?
Correct Answer
C — Accumulated endogenous organic anions compete with furosemide at proximal tubule transporters, reducing luminal drug delivery
Rationale
Loop diuretics reach the tubular lumen not by glomerular filtration but by active secretion through organic anion transporters on the proximal tubule basolateral membrane. In chronic kidney disease, endogenous organic anions accumulate and compete with furosemide at these transporters, reducing luminal drug concentration below the threshold needed for Na-K-2Cl cotransporter blockade. The appropriate clinical response is dose escalation to achieve threshold luminal concentration, not dose reduction.
Question 11
A patient started on furosemide for edema develops hypokalemia. Which of the following best explains the mechanism by which loop diuretics cause potassium loss?
Correct Answer
B — Increased sodium delivery to the collecting duct stimulates potassium secretion via the renal outer medullary potassium channel
Rationale
Loop diuretics block Na-K-2Cl cotransporter-mediated sodium reabsorption in the thick ascending limb, flooding the more distal tubular segments with sodium. The elevated luminal sodium stimulates the epithelial sodium channel in the collecting duct, creating a more electronegative lumen that drives potassium secretion via the renal outer medullary potassium channel. Option D describes the mechanism of thiazide-induced hypokalemia (via secondary hyperaldosteronism), which is a distinct pathway from loop diuretic-induced potassium loss.
Question 12
Severe hyponatremia occurs far more commonly with thiazide diuretics than with loop diuretics. Which of the following best explains this difference?
Correct Answer
D — Thiazides preserve the medullary concentration gradient, allowing antidiuretic hormone to produce concentrated urine and water retention despite sodium loss
Rationale
Loop diuretics disrupt the medullary concentration gradient by blocking Na-K-2Cl cotransporter-mediated sodium reabsorption in the thick ascending limb, producing near-isotonic urine regardless of antidiuretic hormone status. Thiazides act only in the distal convoluted tubule and leave the medullary gradient intact — so a patient on a thiazide can still respond to antidiuretic hormone with highly concentrated urine. In the presence of non-osmotic antidiuretic hormone secretion (pain, nausea, volume depletion), this combination produces profound hyponatremia. This difference in effect on the medullary gradient explains why thiazides pose far greater hyponatremia risk than loop diuretics.
Question 13
A patient on long-term hydrochlorothiazide therapy develops gout. Which of the following best explains the mechanism by which thiazide diuretics cause hyperuricemia?
Correct Answer
A — Competition with urate at organic anion transporters in the proximal tubule reduces urate secretion, and volume contraction upregulates urate reabsorption
Rationale
Loop and thiazide diuretics compete with urate at organic anion transporter sites in the proximal convoluted tubule, reducing secretion of urate into the tubular lumen. Additionally, volume contraction from diuresis upregulates the urate transporter URAT1, increasing urate reabsorption. These two mechanisms act together to raise serum uric acid and precipitate gout in susceptible patients.
Question 14
A patient with heart failure on furosemide begins taking ibuprofen regularly for knee pain. His daily urine output decreases markedly. Which of the following best explains the mechanism by which nonsteroidal anti-inflammatory drugs reduce the efficacy of loop diuretics?
Correct Answer
C — Inhibition of prostaglandin synthesis removes vasodilatory and natriuretic renal prostaglandin tone, reducing glomerular filtration rate and blunting the diuretic response
Rationale
Renal prostaglandins normally oppose tubular sodium reabsorption and dilate the afferent arteriole to maintain glomerular filtration rate. Nonsteroidal anti-inflammatory drugs inhibit cyclooxygenase-dependent prostaglandin synthesis, removing this vasodilatory and natriuretic tone. The result is a reduction in glomerular filtration rate and a blunted natriuretic response to the loop diuretic. This interaction is clinically significant when patients self-medicate with over-the-counter nonsteroidal anti-inflammatory drugs while on diuretic therapy.
Clinical Correlations · Questions 15–18
Apply pharmacological knowledge to clinical scenarios. Each vignette presents a patient situation; the question tests mechanism of action or drug selection.
Question 15
A 58-year-old man is admitted with confusion, fatigue, and a serum calcium of 13.8 mg/dL. He has a history of squamous cell lung cancer. His physician administers aggressive intravenous normal saline to restore intravascular volume. Which of the following drugs should be added to promote urinary calcium excretion based on its mechanism of action?
Correct Answer
B — Furosemide, which abolishes the lumen-positive potential in the thick ascending limb and impairs paracellular calcium reabsorption
Rationale
Loop diuretics are used after volume restoration in hypercalcemia because they abolish the lumen-positive electrical potential in the thick ascending limb, eliminating the driving force for paracellular calcium reabsorption and promoting calciuresis. Thiazides retain calcium and are contraindicated in hypercalcemia. Spironolactone and acetazolamide do not have meaningful effects on renal calcium handling in this context.
Question 16
A 42-year-old woman has had three calcium oxalate kidney stones in the past four years. Her 24-hour urine collection shows elevated urinary calcium excretion. Her physician prescribes a thiazide diuretic to reduce stone recurrence. Which of the following best explains how this drug class reduces urinary calcium in this patient?
Correct Answer
C — Reduced intracellular sodium enhances basolateral sodium-calcium exchange, increasing apical calcium entry through the TRPV5 channel
Rationale
Thiazides block the sodium-chloride cotransporter in the distal convoluted tubule, reducing intracellular sodium in the tubular cell. This enhances basolateral sodium-calcium exchange, which draws calcium from the tubular lumen into the cell through the apical TRPV5 calcium channel. The net result is calcium retention in the body and reduced urinary calcium — the opposite of loop diuretics — making thiazides the pharmacological agent of choice for reducing stone recurrence in calcium nephrolithiasis.
Question 17
A 72-year-old man with heart failure has been well-controlled on furosemide for six months. He begins taking naproxen daily for osteoarthritis and returns to clinic three weeks later with worsening edema and reduced urine output. His furosemide dose has not changed. Which of the following best explains the mechanism by which naproxen reduces the efficacy of furosemide in this patient?
Correct Answer
A — Inhibition of renal prostaglandin synthesis reduces afferent arteriolar dilation and natriuretic tone, lowering glomerular filtration rate and blunting the diuretic response
Rationale
Renal prostaglandins normally maintain afferent arteriolar dilation and oppose tubular sodium reabsorption. Naproxen, like all nonsteroidal anti-inflammatory drugs, inhibits cyclooxygenase-dependent prostaglandin synthesis, removing this vasodilatory and natriuretic support. The resulting fall in glomerular filtration rate and reduced natriuretic tone blunts the response to furosemide. This interaction is a common cause of diuretic resistance and volume overload in patients with heart failure who self-medicate with nonsteroidal anti-inflammatory drugs.
Question 18
A 76-year-old woman is brought to the emergency department after her family noted several days of confusion and lethargy. She takes hydrochlorothiazide for hypertension. Her serum sodium is 118 mEq/L. Which of the following best explains why thiazide diuretics, but not loop diuretics, commonly cause severe hyponatremia?
Correct Answer
D — Thiazides preserve the medullary concentration gradient, so antidiuretic hormone can still produce concentrated urine and water retention despite ongoing sodium loss
Rationale
Loop diuretics disrupt the medullary concentration gradient by blocking the Na-K-2Cl cotransporter in the thick ascending limb, producing near-isotonic urine regardless of antidiuretic hormone levels. Thiazides act only in the distal convoluted tubule and leave the medullary gradient intact. A patient on a thiazide with non-osmotic antidiuretic hormone stimulation — from volume depletion, pain, nausea, or other causes — will continue to produce maximally concentrated urine, retaining free water while losing sodium. Elderly women on thiazides represent the classic high-risk group for this severe and potentially life-threatening hyponatremia.