**Core Concept**
The **micturition reflex** is a complex process involving the coordination of the **central nervous system** and the **lower urinary tract**. The first urge to urinate, also known as the **first desire to void**, occurs when the **bladder** is filled to a certain volume. This is mediated by **stretch receptors** in the bladder wall.

**Why the Correct Answer is Right**
The first urge to urinate typically occurs when the bladder contains about **150-200 ml** of urine. At this point, the **stretch receptors** in the bladder wall are activated, sending signals to the **spinal cord** and eventually to the **brain**, where the sensation of the need to urinate is perceived.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the first urge to urinate occurs at a lower volume.
**Option B:** This option is also incorrect as it is lower than the typical volume for the first urge.
**Option D:** This option is incorrect because it represents a volume that is typically associated with a stronger urge to urinate.

**Clinical Pearl / High-Yield Fact**
The first urge to urinate is an important clinical sign, as it can indicate the **integrity of the micturition reflex**.

**Correct Answer:** B. 150 ml