**Core Concept**
The Kveim skin test is a diagnostic tool used to detect tuberculosis (TB) infection. It involves injecting a suspension of Mycobacterium tuberculosis into the skin to observe the local immune response. This test is particularly useful in cases where the diagnosis is uncertain or in patients who are unable to produce a positive result on the Mantoux test.

**Why the Correct Answer is Right**
The Kveim skin test relies on the immune system's ability to recognize and respond to M. tuberculosis. When the pathogen is introduced into the skin, a localized immune response occurs, characterized by the formation of granulomas and the accumulation of immune cells. This reaction is indicative of a past or present TB infection. The test is often used in conjunction with other diagnostic methods, such as chest X-rays and sputum analysis, to confirm the diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Kveim skin test is not used for diagnosing sarcoidosis, which is a different condition characterized by the formation of non-caseating granulomas in various organs.
**Option B:** This option is incorrect because the tuberculin skin test (Mantoux test) is a different diagnostic tool used to detect M. tuberculosis infection, but it involves injecting tuberculin into the skin, not a suspension of the bacteria.
**Option C:** This option is incorrect because the Kveim skin test is not used for diagnosing leprosy, which is a chronic bacterial infection caused by Mycobacterium leprae.

**Clinical Pearl / High-Yield Fact**
The Kveim skin test is a useful diagnostic tool in cases where the Mantoux test is inconclusive or in patients who have been vaccinated with the BCG vaccine. However, it is essential to note that the test is not widely available and has largely been replaced by more modern diagnostic methods, such as PCR and interferon-gamma release assays.

**Correct Answer: C.**