**Core Concept**
Leprosy diagnosis involves the use of various tests to detect the bacterium *Mycobacterium leprae*. The disease presents with a wide range of clinical manifestations, making accurate diagnosis crucial for effective treatment and prevention of complications.

**Why the Correct Answer is Right**
The correct answer is the test that is not used for the diagnosis of leprosy. Leprosy diagnosis primarily relies on clinical evaluation, histopathological examination of skin and nerve biopsies, and immunological tests such as the lepromin test. The lepromin test, also known as the Mitsuda test, measures the body's immune response to *M. leprae* and is used to differentiate between lepromatous and tuberculoid leprosy. However, other tests are not used for leprosy diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** The lepromin test is indeed used for leprosy diagnosis, making this option incorrect.
**Option B:** The Mantoux test is used for tuberculosis diagnosis, not leprosy, and is also not relevant here since we're looking for a test not used for leprosy.
**Option C:** The Venereal Disease Research Laboratory (VDRL) test is used for syphilis diagnosis, not leprosy, making this an incorrect option.
**Option D:** The FTA-ABS test is used for syphilis diagnosis, not leprosy, making this an incorrect option.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that leprosy diagnosis requires a combination of clinical evaluation, histopathological examination, and immunological tests, making it a challenging condition to diagnose.

**Correct Answer: D.** FTA-ABS test is not used for the diagnosis of leprosy.