**Core Concept**
The presence of renal artery stenosis can lead to decreased blood flow to the kidney, triggering the formation of collateral blood vessels to compensate for the reduced perfusion. This adaptation is a critical sign of the stenosis's clinical significance.

**Why the Correct Answer is Right**
The correct answer is the presence of collaterals, which indicates that the body has attempted to bypass the narrowed segment of the renal artery. This is often a sign of significant stenosis, as the body tries to maintain perfusion to the kidney. The development of collaterals is a result of increased renin release from the stenotic kidney, leading to increased angiotensin II levels, which in turn stimulates the growth of collateral vessels.

**Why Each Wrong Option is Incorrect**
**Option A:** A percentage diameter stenosis >70% is a common threshold used to define significant stenosis, but it's not the most important sign. Some patients may have severe stenosis without significant clinical consequences, whereas others may have mild stenosis with pronounced symptoms.

**Option C:** A systolic pressure gradient >20 mmHg across the lesion can be measured during angiography, but it's not a reliable indicator of clinical significance. This gradient may not accurately reflect the severity of the stenosis or its impact on renal function.

**Option D:** Post-stenotic dilatation of the renal artery is often seen in cases of significant stenosis, but it's not a definitive sign of clinical significance. This dilatation can occur due to various factors, including turbulence of blood flow or changes in vascular wall compliance.

**Clinical Pearl / High-Yield Fact**
When interpreting renal angiograms, remember that the presence of collaterals is a more reliable indicator of significant renal artery stenosis than percentage diameter stenosis or pressure gradients.

**✓ Correct Answer: B. Presence of collaterals**