**Core Concept**
Physiological uncouplers are substances that disrupt the normal coupling between the electron transport chain and the production of ATP in mitochondria. This process leads to the generation of heat instead of ATP, which is crucial for thermogenesis in certain tissues.

**Why the Correct Answer is Right**
All three options, thyroxine, free fatty acids, and thermogenin, are physiological uncouplers that play a vital role in thermogenesis. Thyroxine, a hormone produced by the thyroid gland, increases the permeability of the mitochondrial inner membrane to protons, leading to increased heat production. Free fatty acids, such as palmitate and oleate, also act as uncouplers by increasing the mitochondrial inner membrane permeability, thereby generating heat. Thermogenin, also known as uncoupling protein 1 (UCP1), is a protein found in brown adipose tissue that acts as a proton channel, facilitating the transfer of protons across the mitochondrial inner membrane and leading to heat production.

**Why Each Wrong Option is Incorrect**
**Option A:** Thyroxine is indeed an uncoupler, but the question asks for a physiological uncoupler. While thyroxine has uncoupling effects, it is primarily a hormone involved in regulating metabolism and growth.
**Option B:** Free fatty acids are indeed physiological uncouplers, but the question is asking for all options that fit the description, not just one of them.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that physiological uncouplers play a crucial role in thermogenesis, which is the process of generating heat in response to cold exposure. This process is vital for maintaining body temperature and is primarily mediated by brown adipose tissue.

**Correct Answer:**
✓ Correct Answer: D. All of the above