**Core Concept**
The International Prognostic Index (IPI) is a scoring system used to predict the outcome of patients with aggressive non-Hodgkin lymphoma (NHL). It takes into account several clinical and laboratory parameters to estimate the patient's prognosis.

**Why the Correct Answer is Right**
The IPI includes five prognostic factors: age (≥ 60 years), Ann Arbor stage (III-IV), the number of extranodal sites (≥ 2), the presence of systemic symptoms (such as fever, night sweats, or weight loss), and the type of lymphoma (low or high grade). These factors are used to calculate the patient's risk category, which determines the treatment approach and prognosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Age (≥ 60 years) is actually a correct component of the IPI, so this option is incorrect because it is not the answer we're looking for.
**Option B:** Ann Arbor stage (III-IV) is a correct component of the IPI, so this option is incorrect because it is not the answer we're looking for.
**Option C:** The presence of systemic symptoms (such as fever, night sweats, or weight loss) is a correct component of the IPI, so this option is incorrect because it is not the answer we're looking for.

**Clinical Pearl / High-Yield Fact**
The IPI is an important tool for clinicians to assess the prognosis of patients with aggressive NHL and to guide treatment decisions. It's essential to remember that the IPI is not a perfect predictor, and individual patient factors can influence the outcome.

**Correct Answer: D**