**Question:** A positive tuberculin test is indicated by an area of induration of

A. 0-5 mm
B. 5-9 mm
C. 10-14 mm
D. ≥15 mm

**Correct Answer:** D. ≥15 mm

**Core Concept:**
Tuberculin tests, also known as Mantoux tests, are used to diagnose latent Mycobacterium tuberculosis infection. The test involves injecting purified protein derivative (PPD) into the skin to stimulate a localized immune response. The size of the induration (swelling) is measured as the key indicator of the immune response.

**Why the Correct Answer is Right:**
The test results are interpreted based on the size of the induration. A positive test is defined as an area of induration ≥15 mm, indicating a strong immune response due to the presence of Mycobacterium tuberculosis antigens.

**Why Each Wrong Option is Incorrect:**
A positive tuberculin test is not indicated by the areas of induration in options A, B, and C. These options represent weaker immune responses, which may not necessarily indicate the presence of active tuberculosis or latent infection.

**Why the Correct Answer is Right:**
A positive tuberculin test is associated with a strong immune response, specifically T cell activation and the release of cytokines, which leads to the formation of an induration ≥15 mm. This strong immune response is indicative of the presence of Mycobacterium tuberculosis antigens and is useful in diagnosing latent tuberculosis infection or in monitoring treatment response.

**Clinical Pearl:**
The tuberculin test is a valuable tool in screening for tuberculosis infection, especially in high-incidence areas. However, interpretation of the test results requires a thorough understanding of the immune response and the associated cut-off values. In clinical practice, the tuberculin test is often combined with other diagnostic methods, such as chest X-ray and clinical assessment, to improve the accuracy of tuberculosis diagnosis.