**Core Concept**
Mesophilic organisms are a group of microorganisms that thrive in moderate temperatures, typically between 20°C and 45°C. This temperature range is ideal for their growth and metabolic activities. Mesophilic organisms are commonly found in various environments, including soil, water, and human skin.

**Why the Correct Answer is Right**
Mesophilic organisms grow optimally in temperatures that are neither too hot nor too cold. This allows them to maintain a stable metabolic rate, which is essential for their survival and proliferation. The optimal temperature range for mesophilic organisms is between 20°C and 45°C, as this range supports their enzymatic activity and cell membrane function. The enzymes involved in their metabolic pathways are also most stable and active within this temperature range.

**Why Each Wrong Option is Incorrect**
**Option A:** Temperatures below 20°C are too cold for mesophilic organisms, as their enzymatic activity and metabolic rate are significantly reduced. This makes it difficult for them to grow and multiply.

**Option B:** Temperatures above 45°C are too hot for mesophilic organisms, as their enzymes are denatured, and their cell membranes are disrupted. This leads to a significant decrease in their growth rate and viability.

**Option C:** Psychrophilic organisms grow best at temperatures below 20°C, whereas thermophilic organisms thrive in temperatures above 45°C. Therefore, this option is incorrect as it does not describe mesophilic organisms.

**Clinical Pearl / High-Yield Fact**
It's essential to note that mesophilic organisms are commonly found in various environments and can cause infections in humans, particularly in wounds, burns, and other compromised tissues. Their growth and proliferation can be controlled using antiseptics and disinfectants that are effective against these microorganisms.

**Correct Answer: B. Temperatures above 45°C are too hot for mesophilic organisms, as their enzymes are denatured, and their cell membranes are disrupted.**