Why would i need a 'renal ultrasound' for high blood pressure at only 21?

Evaluation: High blood pressure at 21 years old is very unusual. A renal ultrasound (same as kidney ultrasound) can look for causes like renal artery stenosis, or an adrenal tumor-- both of which could cause high blood pressure at such a young age.

It is a good start.: High blood pressure can be from many causes. The most difficult to diagnose is something called 'essential hypertension' which basically means 'you have hypertension and there is no other cause found.' you can have hypertension because of narrowing of the kidney (renal) arteries. In your age group this can commonly be from fibromuscular hyperplasia (fmd) and this is fairly easily fixed.

Evaluate kidney size: It is important to evaluate the kidneys to identify causes that can be easily fixed. If one kidney is markedly smaller than the other, it may be producing substances which can cause hypertension. Congenitally small kidneys are different from kidneys that are small because of infection or narrowing of the artery to the kidney. Ultrasound is the easiest and least invasive way to begin.