Q-Bank Breakdown: Renin-angiotensin-aldosterone system — Why Every Answer Choice Matters

You just finished a renal physiology block, you’ve memorized “renin → angiotensin II → aldosterone,” and then a Q-bank question shows up where every option sounds kind of right. This is exactly where Step-style questions live: not in whether you “know RAAS,” but whether you can predict what changes renin, angiotensin II, aldosterone, sodium handling, potassium, acid–base, and efferent vs afferent tone under specific clinical conditions.

Tag: Renal > Renal Physiology

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The Clinical Vignette (Q-bank style)

A 67-year-old man with long-standing hypertension and type 2 diabetes is started on a new medication. Two weeks later, his serum creatinine increases from 1.1 mg/dL to 1.6 mg/dL. His potassium is now 5.7 mEq/L (elevated). Blood pressure is modestly improved. He has no edema. Urinalysis is bland.

Which of the following best explains the rise in creatinine?

Answer choices: A. Constriction of the afferent arteriole due to decreased prostaglandin synthesis
B. Dilation of the efferent arteriole causing decreased glomerular capillary hydrostatic pressure
C. Increased sodium delivery to the macula densa leading to increased renin release
D. Increased aldosterone leading to increased potassium retention
E. Increased angiotensin II leading to preferential afferent arteriolar constriction

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Step-by-Step: What’s the Medication?

The combo of:

• Creatinine rise after starting a med
• Hyperkalemia
• Often in a patient with diabetes/HTN (risk of renal artery stenosis, CKD, low renal reserve)

…screams ACE inhibitor (or ARB).

ACE inhibitors/ARBs reduce angiotensin II effects → less aldosterone → hyperkalemia and a predictable small bump in creatinine, especially in patients who rely on efferent constriction to maintain GFR.

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Correct Answer (B): Efferent Dilation ↓ GFR

B. Dilation of the efferent arteriole causing decreased glomerular capillary hydrostatic pressure

Why it’s correct:

• Angiotensin II normally preferentially constricts the efferent arteriole (at physiologic levels), which helps maintain intraglomerular pressure and GFR, especially when renal perfusion is low.
• ACE inhibitors/ARBs blunt angiotensin II → efferent dilation → ↓ glomerular capillary hydrostatic pressure ($P_{GC}$) → ↓ GFR → ↑ creatinine.

High-yield equation logic:

• Net filtration pressure depends heavily on $P_{GC}$. If $P_{GC}$ drops, filtration drops.

Clinical pearl: A creatinine increase up to ~30% after starting an ACEi/ARB can be acceptable and expected. Bigger jumps should make you think bilateral renal artery stenosis, severe volume depletion, NSAID use, etc.

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RAAS Refresher (The Parts That Actually Get Tested)

What stimulates renin release (JG cells)?

Renin increases with:

• ↓ renal perfusion pressure (afferent arteriole baroreceptor)
• ↓ NaCl delivery to macula densa (e.g., loop diuretics)
• ↑ sympathetic tone via $\beta_1$ receptors

What does angiotensin II do (high-yield effects)?

• Constrics efferent arteriole > afferent (maintains GFR)
• ↑ proximal tubule Na⁺ reabsorption via Na⁺/H⁺ exchanger → ↑ HCO₃⁻ reabsorption
• ↑ aldosterone (zona glomerulosa)
• ↑ ADH release + thirst
• Systemic vasoconstriction

Aldosterone’s key downstream effects (principal + alpha-intercalated cells)

• Principal cells: ↑ ENaC and ↑ Na⁺/K⁺ ATPase → Na⁺ reabsorption, K⁺ secretion
• Alpha-intercalated cells: ↑ H⁺ secretion (via H⁺-ATPase) → tendency toward metabolic alkalosis with aldosterone excess

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Now Destroy the Distractors (Why Each One Is Wrong)

A. Constriction of the afferent arteriole due to decreased prostaglandin synthesis

This is NSAIDs, not ACE inhibitors/ARBs.

• NSAIDs inhibit COX → ↓ prostaglandins
• Prostaglandins normally dilate the afferent arteriole
• So NSAIDs → afferent constriction → ↓ renal blood flow → ↓ GFR → ↑ creatinine

Testable contrast:

• ACEi/ARB: efferent dilation
• NSAIDs: afferent constriction
• Diuretics: volume depletion → ↓ perfusion
  The “triple whammy” (ACEi/ARB + NSAID + diuretic) is a classic setup for AKI.

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C. Increased sodium delivery to the macula densa leading to increased renin release

Backwards physiology.

• Macula densa senses NaCl in the distal tubule (NKCC2 transporter)
• High NaCl delivery → signals that GFR/perfusion is adequate → decreases renin
• Low NaCl delivery (or loop diuretics blocking NKCC2) → increases renin

If the stem had a loop diuretic (e.g., furosemide), that would increase renin by making macula densa think NaCl delivery is low.

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D. Increased aldosterone leading to increased potassium retention

Double wrong:

1. Aldosterone causes potassium secretion, not retention.
2. ACE inhibitors/ARBs decrease aldosterone, which leads to hyperkalemia due to reduced K⁺ secretion.

So the correct relationship is:

• ↓ aldosterone → ↓ ENaC activity → less lumen-negative potential → ↓ K⁺ secretion → hyperkalemia

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E. Increased angiotensin II leading to preferential afferent arteriolar constriction

Wrong direction and wrong vessel.

• ACE inhibitors/ARBs cause decreased angiotensin II, not increased.
• Angiotensin II preferentially constricts the efferent arteriole (especially at lower/moderate levels).
• In severe states, angiotensin II can constrict both, but the classic Step association is efferent > afferent.

If you see “increased angiotensin II,” think:

• efferent constriction
• ↑ proximal Na⁺ reabsorption (Na⁺/H⁺ exchange)
• ↑ aldosterone, ↑ ADH

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Rapid Comparison Table (High-Yield for Q-bank Speed)

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Common Step Traps & How to Avoid Them

1) “Creatinine went up after ACE inhibitor” — panic or expected?

• Mild rise: expected due to efferent dilation lowering $P_{GC}$.
• Big rise, refractory HTN, abdominal bruit: think renal artery stenosis (especially bilateral).

2) Hyperkalemia mechanisms (don’t memorize—predict)

ACEi/ARB → ↓ ang II → ↓ aldosterone → ↓ ENaC/Na⁺ reabsorption → ↓ K⁺ secretion → hyperkalemia.

3) RAAS and acid–base

• Aldosterone excess → ↑ H⁺ secretion → metabolic alkalosis
• Aldosterone deficiency or resistance (type 4 RTA) → metabolic acidosis + hyperkalemia

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Take-Home: The One-Liner You Want on Test Day

ACE inhibitors and ARBs dilate the efferent arteriole, lowering intraglomerular pressure and GFR → creatinine rises; they also lower aldosterone → hyperkalemia.