**Core Concept**
REM (Rapid Eye Movement) sleep is a stage of sleep characterized by low muscle tone, rapid eye movements, and high brain activity, similar to being awake. It plays a crucial role in memory consolidation, learning, and emotional regulation.

**Why the Correct Answer is Right**
During REM sleep, the brain's neurotransmitters, such as norepinephrine, serotonin, and acetylcholine, are deactivated, leading to a disconnection between the thalamus and the cortex. This disconnection results in the characteristic vivid dreams and paralysis of the limbs, known as atonia. The brain's default mode network is also active during REM sleep, which is responsible for mental time travel and self-referential thinking.

**Why Each Wrong Option is Incorrect**
**Option A:** REM sleep is associated with increased muscle tone, which is incorrect as it is characterized by atonia.
**Option B:** REM sleep is the stage where the body is most responsive to external stimuli, which is incorrect as it is actually the stage with the lowest body temperature and slowest heart rate.
**Option C:** REM sleep is the stage where the brain is most active in terms of glucose consumption, which is incorrect as it is actually the slow-wave sleep stage where glucose consumption is highest.

**Clinical Pearl / High-Yield Fact**
REM sleep is essential for memory consolidation, and its disruption can lead to memory impairments and cognitive decline. This is why sleep deprivation can have significant consequences on learning and memory.

**Correct Answer:** C. REM sleep is the stage where the brain's neurotransmitters are deactivated, leading to vivid dreams and paralysis of the limbs.