A 52-year-old man with newly diagnosed hypertension (BP 162/104 mmHg) undergoes investigation. Serum creatinine is 2.8 mg/dL, eGFR 22 mL/min/1.73m², and renal ultrasound shows asymmetric kidney sizes (right 9 cm, left 11.5 cm) with a bruit on abdominal auscultation. Which single finding best discriminates renal artery stenosis from chronic kidney disease as the primary cause of hypertension?

Explanation

## Discriminating Feature: Asymmetric Renal Size + Elevated Renin **Key Point:** Renal artery stenosis (RAS) classically presents with **asymmetric renal atrophy** (stenotic kidney is smaller) combined with **elevated plasma renin activity**, which reflects the ischemic kidney's activation of the renin-angiotensin system. In contrast, chronic kidney disease (CKD) typically causes **symmetric renal shrinkage** with normal or suppressed renin. ### Pathophysiology and Discriminating Features | Feature | Renal Artery Stenosis | Chronic Kidney Disease | |---------|----------------------|------------------------| | **Renal Size** | Asymmetric (stenotic kidney smaller) | Symmetric shrinkage | | **Plasma Renin Activity** | Elevated (ischemia-driven) | Normal to low | | **Mechanism of HTN** | Renin-angiotensin activation (secondary) | Multifactorial (fluid retention, RAAS, sympathetic) | | **Abdominal Bruit** | Present in 50–60% (atherosclerotic RAS) | Absent | | **Onset of HTN** | Often sudden (especially fibromuscular dysplasia) | Gradual | | **Proteinuria** | Mild or absent | Often present (glomerular disease) | | **Serum Creatinine** | May rise acutely after ACE inhibitor | Chronically elevated | **High-Yield:** The **combination of asymmetric renal size + elevated renin** is the single best discriminator. The ischemic kidney in RAS releases renin in response to reduced perfusion pressure, whereas in CKD, bilateral glomerular loss leads to symmetric atrophy and renin suppression. **Clinical Pearl:** In this patient, the **smaller right kidney (9 cm)** with **elevated renin** strongly suggests RAS of the right renal artery. The abdominal bruit further supports atherosclerotic RAS (common in older men with cardiovascular risk factors). ### Why Other Findings Are Non-Discriminatory - **Elevated creatinine and reduced eGFR** occur in both conditions and reflect renal dysfunction, not its cause. - **Abdominal bruit** is present in only 50–60% of atherosclerotic RAS and is absent in fibromuscular dysplasia; it is not a reliable discriminator. - **Hypertension with proteinuria** is common in CKD (glomerular disease) but can also occur in RAS with secondary glomerulosclerosis; not specific. **Mnemonic:** **RAS = Renin + Asymmetry** — if you see a smaller kidney with high renin, think renal artery stenosis.