Final answer:
Undernutrition and infectious disease have a bidirectional relationship, as undernutrition can lead to a weakened immune system and increased vulnerability to infections, while infectious diseases can exacerbate undernutrition by affecting nutrient absorption and increasing nutritional needs.
Step-by-step explanation:
The relationship between undernutrition and infectious disease is bidirectional, meaning that each condition can increase the risk of the other. Therefore, when a client is vulnerable to developing a health problem, the nurse chooses a risk nursing diagnosis.
In option A, undernutrition can compromise the immune system, making an individual more susceptible to infections. Option B explains that having an infectious disease can lead to increased nutritional needs, reduced appetite, and poor absorption of nutrients, which may cause undernutrition. So, both A and B are correct in explaining the interconnected impacts of undernutrition and infectious diseases.
Thus, the correct answer to the question is D. A and B, which states that undernutrition can increase the risk of infectious disease, and infectious disease can increase the risk of undernutrition.