**Question:** Irradiation can be used to sterilize all except:

A. biological tissues
B. vaccines
C. medical devices
D. living organisms

**Core Concept:**
Irradiation is a process that uses ionizing radiation to inactivate microorganisms, viruses, and other pathogens. This method is used for a variety of applications, including sterilization of materials, food preservation, and cancer treatment. However, certain materials and biological entities are resistant to irradiation-induced sterilization due to their unique properties.

**Why the Correct Answer is Right:**
D. living organisms are sterilized by irradiation.

Living organisms possess complex biological structures that can repair DNA damage caused by ionizing radiation. In contrast, inanimate materials and most pathogens are more susceptible to sterilization by irradiation due to their simpler chemical compositions and lack of repair mechanisms. Additionally, irradiation can cause crosslinking and degradation of biological macromolecules, leading to inactivation of living organisms.

**Why Each Wrong Option is Incorrect:**

A. Biological tissues: Although tissues can be damaged by irradiation, they are not effectively sterilized due to their ability to repair radiation-induced DNA damage and their complex cellular structures.

B. Vaccines: Vaccines contain inactivated pathogens, which can be sterilized by irradiation. However, vaccines also contain live or attenuated pathogens, which are sterilized by irradiation but are crucial components of the vaccine's effectiveness.

C. Medical devices: Sterilization of medical devices is achieved through irradiation, ensuring their safety and preventing contamination. However, some devices may require additional steps to ensure complete sterilization, especially those with porous surfaces or intricate designs.

**Clinical Pearl:**

Irradiation can be an effective method for sterilizing materials and inactivating certain pathogens. However, certain materials, vaccines, and living organisms remain resistant to irradiation-induced sterilization due to their complex biological structures and repair mechanisms. Adequate knowledge of these limitations is essential when considering irradiation as a sterilization method in various medical contexts.