The portal of entry is the way that a pathogen enters the body. In this case, we are looking for the option that is NOT a portal of entry. Let's break down each option:
1. **Nose:** The nose is a common portal of entry for pathogens like bacteria and viruses. For example, when someone breathes in contaminated air, the pathogens can enter through the nose and cause infection.
2. **Skin:** While the skin is a barrier that protects the body from many pathogens, it is not an absolute barrier. Pathogens can enter the body through cuts, scrapes, or puncture wounds in the skin.
3. **Open wound:** An open wound is a direct portal of entry for pathogens. When the skin is broken, it provides an entry point for bacteria, viruses, and other microorganisms to enter the body and cause infection.
4. **Mouth:** The mouth is another common portal of entry for pathogens. Pathogens can enter the body through contaminated food, drinks, or objects that are put in the mouth.
Therefore, the option that is NOT a portal of entry among the choices provided is **skin**. While intact skin acts as a protective barrier, when there are openings like cuts or wounds, it can serve as a portal of entry for pathogens.
