**Core Concept**
Leprosy, also known as Hansen's disease, is a chronic bacterial infection caused by *Mycobacterium leprae*. The bacilli primarily target the peripheral nerves and the skin, leading to a wide range of clinical manifestations.

**Why the Correct Answer is Right**
_Mycobacterium leprae_ is an acid-fast bacillus, meaning it retains the stain even after decolorization with acid-alcohol. This characteristic is due to the presence of mycolic acids in its cell wall. The bacilli have a unique growth requirement, being thermosensitive and unable to grow at temperatures above 37°C. This property is exploited in the cultivation of the bacilli, where they are grown in footpads of mice.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the correct characteristic of lepra bacilli.

**Option B:** This option is incorrect because it is not a true statement about lepra bacilli.

**Option C:** This option is incorrect because it does not accurately describe the growth requirements of lepra bacilli.

**Clinical Pearl / High-Yield Fact**
Leprosy is characterized by a wide range of clinical manifestations, from the tuberculoid form with minimal symptoms to the lepromatous form with widespread skin and nerve involvement. Early diagnosis and treatment are crucial to prevent long-term disability.

**Correct Answer: D**