**Core Concept**
A negative Montoux test, also known as a tuberculin skin test (TST), is used to diagnose latent or active tuberculosis (TB) infection. The test measures the body's immune response to tuberculin, a substance derived from the bacteria that cause TB. A positive test indicates that the individual has been exposed to TB.

**Why the Correct Answer is Right**
The correct answer is based on the threshold for a negative Montoux test, which is typically 5 mm or less of induration (hardening of the skin) in a person with no risk factors for TB. Induration is the key measure, not erythema (redness). The test is read 48-72 hours after the injection of tuberculin, and the results are interpreted as follows: 0-4 mm induration is considered a negative result, 5-9 mm is considered indeterminate or negative in some cases, 10-14 mm is considered positive in individuals with no risk factors, and 15 mm or more is considered positive in individuals with risk factors.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the threshold for a negative Montoux test.
**Option B:** This option is incorrect because it does not accurately describe the threshold for a negative Montoux test.
**Option C:** This option is incorrect because it does not provide a clear and specific answer.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the Montoux test is not 100% sensitive and specific, and false-negative results can occur in individuals with latent TB infection or those who have been vaccinated with the bacille Calmette-Guérin (BCG) vaccine.

**Correct Answer: D. 5 mm or less**