## Core Concept
Smooth muscle is a type of non-striated muscle tissue that is found in the walls of hollow organs, such as the digestive tract, blood vessels, and airways. It plays a crucial role in regulating various bodily functions, including blood pressure, digestion, and respiration. Smooth muscle contraction and relaxation are controlled by the autonomic nervous system and various hormones.

## Why the Correct Answer is Right
The correct answer, , states that smooth muscle is not under voluntary control. This is a fundamental characteristic of smooth muscle, as it is primarily controlled by the autonomic nervous system and involuntary mechanisms, unlike skeletal muscle which is under voluntary control.

## Why Each Wrong Option is Incorrect
* **Option A:** If option A states that smooth muscle is found in the walls of hollow organs, this is actually a true statement, not false.
* **Option B:** If option B mentions that smooth muscle contraction is controlled by the autonomic nervous system, this is also a true statement.
* **Option C:** If option C notes that smooth muscle has a single nucleus per cell, this is generally true for smooth muscle cells (fibers).
* **Option D:** If option D claims that smooth muscle is under voluntary control, this would be incorrect as smooth muscle contraction is primarily involuntary.

## Clinical Pearl / High-Yield Fact
A key point to remember is that smooth muscle can exhibit **tonic contraction**, maintaining a constant level of contraction over a long period, which is crucial for maintaining blood pressure and sphincter tone. This characteristic is vital for understanding various physiological and pathological processes.

## Correct Answer: D. under voluntary control