A 52-year-old man with chronic kidney disease (CKD stage 3b) presents with hypertension refractory to two antihypertensive agents. Serum creatinine is 2.8 mg/dL, and urinalysis shows proteinuria. Plasma renin activity is markedly elevated. Which is the most common pathophysiological mechanism underlying secondary hypertension in this patient?

Explanation

## Hypertension in CKD: RAAS-Mediated Pathophysiology ### Mechanism of Secondary Hypertension in CKD **Key Point:** In chronic kidney disease, reduced glomerular filtration rate (GFR) and loss of functional nephrons lead to decreased renal perfusion pressure at the afferent arteriole, triggering renin release and activation of the RAAS — the most common mechanism of hypertension in CKD. ### Pathophysiological Cascade ```mermaid flowchart TD A[Nephron loss in CKD]:::outcome --> B[↓ GFR and renal perfusion pressure]:::outcome B --> C[Afferent arteriole baroreceptor activation]:::decision C --> D[↑ Renin release from JG cells]:::action D --> E[↑ Angiotensin II formation]:::action E --> F[Vasoconstriction + ↑ Aldosterone]:::action F --> G[↑ Na+ reabsorption + ↑ Blood volume]:::action G --> H[Hypertension]:::urgent E --> I[↑ Sympathetic tone]:::action I --> H ``` ### Why RAAS Activation Occurs in CKD | Factor | Effect in CKD | |--------|---------------| | **Reduced functional nephrons** | Fewer glomeruli → lower total GFR | | **Decreased renal perfusion pressure** | Afferent arteriole senses hypoperfusion | | **Impaired sodium excretion** | Tubular dysfunction → macula densa senses ↓ Na^+^ delivery | | **Elevated plasma renin activity** | Compensatory renin release (diagnostic clue) | | **Angiotensin II-mediated vasoconstriction** | Preferential efferent arteriole constriction → maintains GFR (short-term) but worsens hypertension | | **Aldosterone-driven Na^+^ retention** | Expansion of extracellular fluid volume | **High-Yield:** Markedly elevated plasma renin activity in a hypertensive CKD patient is a hallmark of RAAS-mediated hypertension and distinguishes it from primary hypertension or other secondary causes (which typically have suppressed renin). ### Clinical Implications **Clinical Pearl:** ACE inhibitors and ARBs are first-line agents in CKD with hypertension because they block angiotensin II formation and preferentially dilate the efferent arteriole, reducing glomerular capillary pressure and slowing CKD progression — a dual benefit beyond blood pressure control. **Warning:** Do not confuse CKD-related hypertension (RAAS-mediated, high renin) with primary hyperaldosteronism (low renin, high aldosterone) or pheochromocytoma (elevated catecholamines). The clinical context and biochemical profile are diagnostic. ### Why This Is the Most Common Mechanism in CKD - Occurs in >50% of CKD patients by stage 3–4 - Direct consequence of nephron loss and reduced renal perfusion - Pathophysiology is well-established and reproducible - Plasma renin activity elevation is a diagnostic marker [cite:Harrison 21e Ch 280; KD Tripathi 8e Ch 12]