**Core Concept**
The thermoregulatory center in the hypothalamus plays a crucial role in maintaining the body's temperature within a narrow range. Fever is a complex physiological response to infection or inflammation, and it involves the hypothalamic thermoregulatory center.

**Why the Correct Answer is Right**
When the body detects the presence of pyrogens (e.g., endotoxins or cytokines) during an infection, these molecules interact with specific receptors in the hypothalamus, leading to a shift in the thermoregulatory set point. This means that the hypothalamus resets its temperature set point to a higher value, which is perceived as fever. The hypothalamus responds by increasing the body's temperature through various mechanisms, such as vasoconstriction, shivering, and increased metabolic rate. This response is aimed at creating an environment that is less favorable for the growth and replication of pathogens.

**Why Each Wrong Option is Incorrect**
**Option B:** Resetting of skin temperature is not the primary mechanism by which fever is induced. While skin temperature may increase as a result of fever, the underlying cause is the resetting of the hypothalamic thermoregulatory center.

**Option C:** Both of the above is incorrect because only the thermoregulatory center shifting to a new level is the correct explanation for fever. Resetting of skin temperature is not a primary mechanism for inducing fever.

**Option D:** None of the above is incorrect because option A is the correct explanation for fever.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the hypothalamus plays a critical role in temperature regulation, and any disruption in this process can lead to fever. The hypothalamic thermoregulatory center is sensitive to pyrogens, which can cause a shift in the temperature set point, resulting in fever.

**✓ Correct Answer: A. Thermoregulatory centre to shift to new level**