Final answer:
Lymph node involvement is the single most significant prognostic feature in breast cancer.
Step-by-step explanation:
The single most significant prognostic feature in breast cancer is lymph node involvement. Lymph node involvement refers to the spread of cancer cells to nearby lymph nodes, which indicates a higher risk of the cancer spreading further and potential metastasis. This information is crucial for determining the stage of breast cancer and guiding treatment decisions.