**Question:** Sterilisation of media containing serum:

A. Autoclaving
B. Ethanol
C. UV radiation
D. Gamma irradiation

**Core Concept:** Sterilisation is the process of eliminating all forms of microorganisms, including bacteria, viruses, and fungi, from a solution or object to prevent contamination and maintain sterility. In the context of media containing serum, sterilisation is essential for ensuring the safety and efficacy of the culture environment.

**Why the Correct Answer is Right:**

D. Gamma irradiation is the correct method for sterilising media containing serum because it provides a high level of sterility, as it inactivates a wide range of microorganisms effectively. Gamma irradiation uses ionising radiation to damage the nucleic acids, proteins, and cellular structures, rendering microorganisms unable to reproduce. This method is particularly suitable for sterilising liquid media like serum-containing culture media, as it can penetrate liquids without causing significant changes to the physical properties of the medium.

**Why Each Wrong Option is Incorrect:**

A. Autoclaving is a common method for sterilising solid objects, but it is less effective for liquid media due to limited penetration depth of steam. Thus, autoclaving may not adequately sterilise serum-containing culture media.

B. Ethanol is an effective disinfectant but not a sterilisation method. Ethanol kills microorganisms by denaturing their cellular structures, but it does not guarantee the eradication of all microorganisms, especially spores that can survive ethanol treatment.

C. UV radiation is effective for surface sterilisation but not sterilisation of liquids. The penetration depth of UV radiation is limited, and serum-containing culture media may not receive sufficient exposure to ensure complete inactivation of microorganisms.

**Clinical Pearls:**

1. Sterilisation of serum-containing culture media is crucial for maintaining the safety and consistency of cell cultures and microbiological experiments.
2. Gamma irradiation is the preferred method for sterilising liquid media, as it ensures broad-spectrum inactivation of microorganisms without causing significant changes to the serum or media composition.
3. Autoclaving should be avoided for sterilising serum-containing culture media, as it may not provide adequate sterility due to limited penetration depth of steam. Using ethanol or UV radiation for media sterilisation is not recommended because they are not effective against microorganisms in liquids.