**Core Concept**
Sterilization by steam under pressure, also known as autoclaving, is a process used to eliminate microorganisms from medical instruments and equipment. This method utilizes saturated steam at elevated temperatures to denature proteins and disrupt the bacterial cell wall, ultimately leading to cell death. Autoclaving is a widely accepted and effective method for sterilization in healthcare settings.

**Why the Correct Answer is Right**
Autoclaving works by creating a high-pressure environment where steam is in contact with the instrument or equipment for a sufficient duration. The heat from the steam denatures proteins, disrupts the bacterial cell wall, and ultimately leads to cell death. The steam also penetrates the instrument or equipment, ensuring that all areas are exposed to the sterilizing effect. The combination of high temperature and pressure makes autoclaving an effective method for sterilization.

**Why Each Wrong Option is Incorrect**
**Option A:** Autoclaving uses dry heat instead of steam.
* This is incorrect because autoclaving specifically utilizes steam under pressure, not dry heat.

**Option B:** Autoclaving is a method of pasteurization.
* This is incorrect because pasteurization involves heating liquids to a lower temperature for a shorter duration, whereas autoclaving uses high-pressure steam to achieve sterilization.

**Option C:** Autoclaving is a method of cold sterilization.
* This is incorrect because autoclaving involves the use of high-temperature steam, not cold temperatures, to achieve sterilization.

**Option D:** Autoclaving is a method of chemical sterilization.
* This is incorrect because autoclaving uses physical means (high-temperature steam) to achieve sterilization, not chemical agents.

**Clinical Pearl / High-Yield Fact**
One key point to remember is that autoclaving is effective against all types of microorganisms, including bacterial spores, which are highly resistant to sterilization methods. This makes autoclaving a crucial step in ensuring the safety of medical instruments and equipment.

**Correct Answer: D. Autoclaving is a method of physical sterilization.