Final answer:
A large, red welt after a tuberculin skin test indicates a positive result, confirming exposure to tuberculosis and an immune response to it.
Step-by-step explanation:
A large, red welt after a tuberculin skin test indicates a positive result. This means that the patient has been exposed to the bacteria that causes tuberculosis and has a cellular immune response to it.
The tuberculin test involves injecting bacterial proteins from M. tuberculosis into the skin. A positive test is indicated by a raised red area, called an induration, which is caused by an accumulation of activated macrophages.
Therefore, a large, red welt after a tuberculin skin test indicates exposure to tuberculosis and an immune response to it.