A 79-year-old man who is being treated with GnRH antagonist for proven adenocarcinoma of the prostate attends a follow up session. What is the most appropriate investigation?

The key is B. Serum PSA. [Serum PSA is used to assess the progress of treatment. (If a man’s PSA level rises after prostate cancer treatment, his doctor will consider a number of factors before recommending further treatment. Additional treatment based on a single PSA test is not recommended. Instead, a rising trend in PSA level over time in combination with other findings, such as an abnormal result on imaging tests, may lead a man’s doctor to recommend further treatment)].