Actually there are a lot of things which can cause blood to be present in the semen, ill list the most common ones below,
1) Strenous exercise - if you have been exercising quite a lot recently or exerting yourself, it can cause blood to appear in the semen and would resolve on its own
2) Sexual intercourse - particulary vigorous intercourse can cause blood to appear in the semen transiently and this again would disappear on its own
3) Infections - there are several infections which can cause this, prostatitis and epididymitis for example. they would also cause burning urination, increased frequency and dark colored urine. A urine examination and clinical examination would diagnose the problem and then you would be treated with antibiotics
4) Prostate enlargement - if you experience urinary hesistancy, poor stream, frequency and nocturnal urination then it is possible that you could have benign enlargement of the prostate. This would be diagnosed with a simple ultrasound and clinical examination.
5) prostate cancer - this is the least likely of all of the above simply because of your age. However if the blood is persistent for more than one to two weeks, then a PSA level and ultrasound would have to be done to rule this out.
For now i would suggest that you increase your fluid intake and see if that helps, especially if you have been exercising or had vigorous intercourse recently. If not then look for the other symptoms i have mentioned and contact your doctor for additional testing. In most cases this does resolve on its own.
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