**Core Concept**
Entropy in a biological system refers to the measure of disorder or randomness in the system. In thermodynamics, entropy is a fundamental concept that describes the spontaneous tendency of systems to become more disordered or random over time, leading to an increase in entropy. However, in biological systems, entropy is maintained at a relatively constant level due to the continuous input of energy from the environment.

**Why the Correct Answer is Right**
The correct answer is related to the concept of homeostasis, which is the ability of biological systems to maintain a stable internal environment despite changes in the external environment. This is achieved through the continuous input of energy from the environment, which is used to maintain the structural and functional integrity of the system. In other words, the energy input helps to counteract the spontaneous increase in entropy, maintaining the system in a state of relative order. This is achieved through the action of enzymes, such as ATPases, which use energy from ATP to maintain the structural and functional integrity of the system.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the concept of entropy in biological systems.
**Option B:** This option is not accurate, as entropy in biological systems does not decrease.
**Option C:** This option is incorrect, as entropy in biological systems does not remain constant due to the continuous input of energy.

**Clinical Pearl / High-Yield Fact**
The concept of homeostasis is crucial in maintaining the stability of biological systems. Understanding the mechanisms that regulate homeostasis is essential in understanding the pathophysiology of various diseases.

**Correct Answer: D. Entropy in a biological system remains constant due to the continuous input of energy from the environment.**