**Core Concept**
The Lepromin test is a skin test used to assess the immune response to leprosy antigens. It measures the body's ability to mount a cell-mediated immune response against Mycobacterium leprae, the causative agent of leprosy. The test is particularly useful in differentiating between tuberculoid and lepromatous leprosy.

**Why the Correct Answer is Right**
A positive Lepromin test indicates a strong cell-mediated immune response, which is characteristic of tuberculoid leprosy. In this form of leprosy, the immune system is able to contain the infection, and the test will show a positive reaction. This is because the body's T-cells are able to recognize and respond to the leprosy antigens, leading to the formation of a skin lesion at the injection site. The test is often used to predict the outcome of leprosy treatment and to differentiate between different forms of the disease.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Lepromin test is not typically used to diagnose lepromatous leprosy, which is characterized by a weak cell-mediated immune response. In lepromatous leprosy, the immune system is unable to contain the infection, and the test will often be negative.
**Option B:** This option is incorrect because the Lepromin test is not a diagnostic tool for borderline leprosy, which is a form of the disease that falls between tuberculoid and lepromatous leprosy in terms of immune response.
**Option C:** This option is incorrect because the Lepromin test is not used to diagnose other forms of mycobacterial infections, such as tuberculosis.

**Clinical Pearl / High-Yield Fact**
The Lepromin test is a useful tool in differentiating between tuberculoid and lepromatous leprosy, and it can help predict the outcome of leprosy treatment. However, it is essential to note that the test is not 100% accurate, and a negative result does not rule out tuberculoid leprosy.

**Correct Answer:** C.