**Core Concept**
The Gleason score is a system used to grade the prognosis of prostate cancer based on the tumor's histological appearance. It takes into account the grade of the two most prevalent patterns of cancer cells, with a higher score indicating a worse prognosis.

**Why the Correct Answer is Right**
The Gleason score was developed by Dr. Donald Gleason in the 1960s and is based on the combination of two grades, ranging from 1 to 5. The score is calculated by adding the two grades, resulting in a total score between 2 and 10. A higher Gleason score indicates a more aggressive tumor with a poorer prognosis. The score is used in conjunction with other factors, such as tumor stage and PSA levels, to determine the overall prognosis and guide treatment decisions.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Gleason score is not used for other types of cancer, such as breast or lung cancer.
**Option B:** This option is incorrect because the TNM staging system, not the Gleason score, is used to determine the extent of cancer spread.
**Option C:** This option is incorrect because the PSA level is an important prognostic factor for prostate cancer, but it is not the Gleason score.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the Gleason score is a prognostic tool, not a diagnostic tool. A high Gleason score indicates a worse prognosis, but it does not necessarily mean that the cancer is more aggressive.

**Correct Answer: D. Prostate cancer.**