Choice A rationale:
Skin and mucous membranes are the most effective and crucial barriers to infection. They provide a continuous physical barrier that prevents pathogens from entering the body. Here's a detailed explanation of their protective mechanisms:
1. Physical Barrier:
Skin: The outermost layer of skin, the epidermis, is composed of tightly packed cells that are difficult for pathogens to penetrate. It's also covered in a layer of sebum, an oily substance that helps to repel water and microorganisms.
Mucous membranes: These moist linings cover the openings of the body, such as the nose, mouth, eyes, and digestive, respiratory, and urogenital tracts. They produce mucus, a sticky substance that traps pathogens and prevents them from entering the body. Mucus also contains enzymes and antibodies that can kill certain pathogens.
2. Chemical Barrier:
Skin and mucous membranes secrete a variety of substances that have antimicrobial properties. These include: Sebum: Contains fatty acids that can kill bacteria and fungi.
Sweat: Contains salt and lysozyme, an enzyme that can break down bacterial cell walls. Saliva: Contains enzymes that can break down food and kill bacteria.
Gastric acid: The highly acidic environment of the stomach kills most pathogens that are ingested.
3. Immune Barrier:
Skin and mucous membranes are home to a diverse community of microbes, known as the microbiome. These microbes play an important role in protecting against infection by competing with pathogens for resources and space.
Mucous membranes contain specialized immune cells, such as M cells and dendritic cells, that can recognize pathogens and initiate an immune response.
In contrast, the other choices are less effective barriers to infection:
Choice B: Gastrointestinal secretions, such as gastric acid, do play a role in preventing infection, but they are not as effective as skin and mucous membranes. Pathogens can still enter the body through the digestive tract, even in the presence of gastric acid.
Choice C: Colonization by host bacteria can actually help to protect against infection by competing with pathogens. However, it is not a primary barrier to infection.
Choice D: Inflammatory processes are a response to infection, not a barrier to it. They occur after pathogens have already entered the body.

Rationale for Choice A:
Documentation is essential for communication and continuity of care, but it is not the most immediate priority in this situation.
The nurse should document the episode of vomiting, including the time, amount, and characteristics of the vomitus, as well as any associated symptoms or interventions.
However, auscultating lung sounds should be done first to assess for potential aspiration, which is a more urgent concern.
Rationale for Choice B:
Offering dry toast may be appropriate after the nurse has assessed for aspiration and determined that it is safe for the client to resume oral intake.
However, it is not the most important action at this time.
The nurse should first assess the client's respiratory status and address any potential complications.
Rationale for Choice C:
Rest is important for healing and recovery, but it is not the most immediate priority in this situation. The nurse should first assess the client's respiratory status and address any potential complications. Once the client is stable, the nurse can then encourage rest.
Rationale for Choice D:
Auscultating lung sounds is the most important action for the nurse to take after a client vomits.
This is because aspiration of vomitus is a serious complication that can lead to pneumonia, respiratory distress, and even death.
By auscultating lung sounds, the nurse can assess for signs of aspiration, such as crackles, wheezing, or diminished breath sounds.
If aspiration is suspected, the nurse can initiate appropriate interventions, such as suctioning, oxygen therapy, and positioning the client to facilitate drainage of secretions.