**Question:** BCG is -

A. Tuberculosis vaccine
B. Chemotherapy drug
C. Bacillus Calmette-Guérin
D. Biopsy collection device

**Correct Answer:** C. Bacillus Calmette-Guérin

**Core Concept:**
BCG stands for Bacillus Calmette-Guérin, which is a live attenuated strain of Mycobacterium bovis. It is primarily used as a vaccine for preventing tuberculosis, a disease caused by the Mycobacterium tuberculosis bacteria. BCG vaccine is effective against disseminated forms of tuberculosis in children but has limited efficacy in preventing pulmonary tuberculosis in adults.

**Why the Correct Answer is Right:**
BCG is a live attenuated strain of Mycobacterium bovis, which has been modified to lose its virulence but retain its ability to stimulate the immune system. This makes it an effective vaccine for tuberculosis. It works by inducing a strong cellular immune response, particularly the production of macrophages and lymphocytes, which play a crucial role in fighting intracellular pathogens.

**Why Each Wrong Option is Incorrect:**

A. Although BCG is primarily used as a vaccine, it is not a tuberculosis drug as it is not used to treat established tuberculosis infection.

B. BCG is not a chemotherapy drug, as chemotherapy involves the use of drugs to treat cancer. BCG is a vaccine to prevent tuberculosis.

D. A biopsy collection device is an instrument used for obtaining tissue samples for histopathological examination. BCG is a live attenuated mycobacterial strain used for vaccination, not a device for collecting biopsies.

**Clinical Pearl:**

BCG vaccination is a part of the routine immunization schedule in many countries, particularly in those with high tuberculosis prevalence. A strong cellular immune response induced by BCG helps prevent disseminated tuberculosis in children and reduces the risk of miliary tuberculosis. While BCG vaccine is not as effective in preventing pulmonary tuberculosis in adults, it remains an essential component of tuberculosis control measures globally.