What diagnostic test is used to identify positive contact with the tuberculosis bacilli?

The injection of purified protein derivative (PPD) is the correct choice for identifying positive contact with the tuberculosis bacilli. This test is also known as the tuberculin skin test or Mantoux test. It involves the intradermal injection of a small amount of PPD, a protein derived from the tuberculosis bacterium. If a person has been exposed to the tuberculosis bacilli, their immune system will respond to the PPD. This reaction usually appears as a raised, hardened area at the injection site within 48 to 72 hours, indicating that the person's immune system has recognized the proteins as foreign, suggesting possible exposure to tuberculosis.

The other diagnostic tests mentioned do not serve the purpose of identifying exposure to tuberculosis. Liver function tests assess the health of the liver and help diagnose liver diseases; prolonged partial thromboplastin time is a coagulation test used to evaluate blood clotting disorders; and pulmonary function tests measure how well the lungs are working, which is not specific to tuberculosis exposure. Each of these tests has its own significance in diagnosing various health conditions, but they do not provide information about an individual's contact with tuberculosis bacilli.