A 62-year-old man with chronic kidney disease undergoes imaging for suspected renal artery stenosis. Which single imaging feature best distinguishes atherosclerotic renal artery stenosis (ARAS) from fibromuscular dysplasia (FMD)?

Explanation

## Distinguishing ARAS from FMD on Imaging ### Pathologic and Imaging Basis **Key Point:** The **'string of beads' appearance** (alternating stenosis and dilatation) is pathognomonic for fibromuscular dysplasia and is the single best discriminator from atherosclerotic renal artery stenosis. ### Comparison Table | Feature | Atherosclerotic RAS (ARAS) | Fibromuscular Dysplasia (FMD) | | --- | --- | --- | | **Location** | Proximal third (ostial) | Mid to distal third | | **Appearance** | Smooth, eccentric stenosis | **'String of beads'** (alternating stenosis/dilatation) | | **Calcification** | Often present | Rare | | **Age of onset** | >50 years, atherosclerotic risk factors | <50 years, female predominance | | **Associated findings** | Aortic atherosclerosis | No systemic atherosclerosis | | **Pathology** | Intimal hyperplasia + smooth muscle proliferation | Medial dysplasia (most common type) | **High-Yield:** The **'string of beads' pattern is 90% specific for FMD** and is virtually never seen in atherosclerotic disease. It represents alternating zones of medial dysplasia and normal vessel wall. ### Why Other Features Fail to Discriminate 1. **Proximal location:** ARAS typically occurs at the ostium (proximal third), but FMD can also involve proximal segments in 10% of cases. 2. **Calcification:** While more common in ARAS, it is not sensitive or specific enough to reliably distinguish the two. 3. **Elevated peak systolic velocity:** Both conditions cause hemodynamically significant stenosis and will show elevated velocities on Doppler; this reflects degree of stenosis, not etiology. **Clinical Pearl:** A young woman (<45 years) with hypertension and no atherosclerotic risk factors presenting with renal artery stenosis should raise suspicion for FMD. The 'string of beads' appearance on CT or MR angiography confirms the diagnosis. **Mnemonic:** **FMD = "Fancy Medial Dysplasia"** → **F**ascinating **M**ultiple **D**ilatations (string of beads).