Final answer:
The most urgent symptom to report for a patient taking tamoxifen is the presence of vaginal bleeding since this could indicate serious complications.
Step-by-step explanation:
The nurse assessing a patient on tamoxifen for breast cancer treatment should report the presence of vaginal bleeding to the provider as a priority. This symptom may indicate a potentially serious side effect of tamoxifen or possible complications such as endometrial changes including cancer, which requires immediate attention. While symptoms like improved appetite, complaints of joint pain, and enhanced sense of taste might be notable, they are not as immediately concerning as vaginal bleeding in the context of tamoxifen therapy.