**Core Concept**
The myogenic theory of renal autoregulation proposes that changes in renal blood pressure are sensed by stretch receptors in the afferent arterioles, leading to a reflexive contraction or dilation to maintain a constant glomerular filtration rate. This is one of the two primary mechanisms of renal autoregulation, the other being the tubuloglomerular feedback mechanism.

**Why the Correct Answer is Right**
When the afferent arteriole is stretched due to increased blood pressure, it activates mechanoreceptors, which trigger an influx of calcium ions (Ca2+) into the smooth muscle cells of the arteriole. This increase in intracellular calcium concentration leads to muscle contraction, causing the afferent arteriole to constrict and reduce blood flow to the glomerulus. The opening of Ca2+ channels is the key step in the myogenic response, as it allows the influx of calcium ions necessary for muscle contraction.

**Why Each Wrong Option is Incorrect**
**Option A:** Nitric oxide (NO) is actually a vasodilator and plays a key role in the tubuloglomerular feedback mechanism, not the myogenic response. It helps to relax smooth muscle cells and increase blood flow.

**Option B:** Noradrenaline is a vasoconstrictor that can also contribute to renal vasoconstriction, but it is not directly involved in the myogenic response to stretch. Noradrenaline acts through alpha-adrenergic receptors to increase intracellular calcium and induce vasoconstriction.

**Option D:** Adenosine release is associated with the tubuloglomerular feedback mechanism and helps to regulate renal blood flow by inducing vasodilation. It is not directly involved in the myogenic response to stretch.

**Clinical Pearl / High-Yield Fact**
The myogenic theory of renal autoregulation is critical in maintaining a constant glomerular filtration rate despite changes in systemic blood pressure. This is particularly important in conditions such as hypertension, where the myogenic response helps to protect the kidneys from excessive pressure.

✓ Correct Answer: C. Opening of Ca2+ channels