**Core Concept**
Leprosy, a chronic bacterial infection caused by *Mycobacterium leprae*, primarily affects the peripheral nerves, skin, and mucous membranes. The disease leads to progressive damage to the peripheral nerves, resulting in sensory and motor deficits.

**Why the Correct Answer is Right**
In leprosy, the earliest sensation lost is typically pain and temperature sensation due to involvement of the small nerve fibers. The damage to these fibers is a result of the bacterial infection, which disrupts the normal functioning of the nerve cells. The loss of pain and temperature sensation is often the first noticeable symptom, as it is mediated by the smallest nerve fibers, which are more susceptible to damage. This disruption affects the spinothalamic tract, which is responsible for transmitting pain and temperature sensation from the body to the brain.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not a correct choice as the earliest sensation lost in leprosy is not proprioception (position sense). Proprioception is mediated by larger nerve fibers, which are less affected in the early stages of the disease.
* **Option B:** This option is incorrect as the earliest sensation lost in leprosy is not vibration sense. Vibration sense is also mediated by larger nerve fibers, which are less affected in the early stages of the disease.
* **Option C:** This option is not a correct choice as the earliest sensation lost in leprosy is not two-point discrimination. Two-point discrimination is a function of the larger nerve fibers, which are less affected in the early stages of the disease.

**Clinical Pearl / High-Yield Fact**
In leprosy, the loss of pain and temperature sensation is often accompanied by a loss of sweating over the affected area, as the sympathetic nerves that control sweating are also damaged. This can lead to a decrease in skin temperature and a characteristic "cool to touch" sensation over the affected area.

**Correct Answer: D. Pain and temperature sensation.**