**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. During REM sleep, dreams occur and the brain processes and consolidates memories.

**Why the Correct Answer is Right**
The correct answer is related to the unique physiological features of REM sleep. Brain activity during REM sleep is similar to that of being awake, with the release of neurotransmitters like norepinephrine, serotonin, and acetylcholine being suppressed. This suppression of neurotransmitters leads to the characteristic features of REM sleep, such as low muscle tone and vivid dreams. The brain's default mode network, which is responsible for self-referential thinking and mind-wandering, is also active during REM sleep, contributing to the vivid nature of dreams.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe a unique feature of REM sleep. While it is true that REM sleep is characterized by low muscle tone, this is not a distinguishing feature of REM sleep compared to other stages of sleep.
**Option B:** This option is incorrect because it is not a characteristic of REM sleep. REM sleep is actually associated with increased brain activity, not decreased activity.
**Option C:** This option is incorrect because it is not a unique feature of REM sleep. While REM sleep is associated with vivid dreams, this is not the only stage of sleep where dreams occur.

**Clinical Pearl / High-Yield Fact**
One important clinical correlation to remember is that REM sleep is essential for memory consolidation and learning. Studies have shown that sleep deprivation, especially during REM sleep, can impair memory and cognitive function.

**Correct Answer: A. Low muscle tone.**