**Core Concept**
The **renin-angiotensin-aldosterone system (RAAS)** plays a crucial role in regulating blood pressure and electrolyte balance. **Renin** is an enzyme that initiates this cascade. The secretion of renin is primarily controlled by the **juxtaglomerular cells**.

**Why the Correct Answer is Right**
Renin is secreted by the **juxtaglomerular cells**, which are specialized smooth muscle cells located in the walls of the **afferent arterioles** in the kidneys. These cells sense decreases in blood pressure and respond by releasing renin, which then triggers the conversion of **angiotensinogen** to **angiotensin I**.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it is not related to the RAAS system.
**Option B:** Incorrect as it does not accurately describe the source of renin secretion.
**Option C:** Incorrect because, although related to the kidney, it is not the specific site of renin secretion.
**Option D:** Incorrect as it is not a recognized source of renin.

**Clinical Pearl / High-Yield Fact**
Remember, the RAAS system is a key regulator of blood pressure, and alterations in renin secretion can lead to hypertension or hypotension, making it a critical concept in nephrology and cardiology.

**Correct Answer:** D. Juxtaglomerular cells.