**Core Concept**
The term 'case' in the context of tuberculosis (TB) refers to a single unit of infection that involves the lung or other parts of the body. This unit of infection is crucial for understanding the transmission dynamics and treatment outcomes of TB.

**Why the Correct Answer is Right**
A 'case' in TB is defined as a person with a newly diagnosed pulmonary or extrapulmonary TB infection, who is not a treatment failure, and has not been previously treated for TB. The World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) use this definition to track TB cases. This definition ensures that only newly diagnosed cases are included in TB surveillance, allowing for accurate assessment of TB prevalence and trends.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify that the case must be newly diagnosed, which is essential for accurate TB surveillance.
**Option B:** This option is incorrect because it does not exclude treatment failures, which can skew TB prevalence estimates.
**Option C:** This option is incorrect because it does not specify that the case must involve a pulmonary or extrapulmonary TB infection, which is necessary for accurate classification of TB cases.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the definition of a 'case' in TB is critical for understanding TB epidemiology and guiding public health interventions. Accurate case definition ensures that TB control programs target newly diagnosed cases, thereby reducing the risk of transmission and improving treatment outcomes.

**Correct Answer: D. A person with a newly diagnosed pulmonary or extrapulmonary TB infection, who is not a treatment failure, and has not been previously treated for TB.**