Brief Answer:
not a great concern but may cause minor complications

Detailed Answer:
Hi and Welcome to Ask a Doctor service. I have read your question and here is my advice.
Medulary nephrocalcinosis means calcium depositions in medula ( part Of kidney) of the kidney. While there may be other causes for it, like high calcium or high phosphate, but more common causes of calcium deposition in medulla is the Medulary Sponge Disease which is your case. (as given by the ultrasound reports).

Medulary Sponge disease is a kidney disease in which the collecting duct (many small ducts which collects urine from different parts of kidneys to transfer it to the bladder) are slightly dilated. The cause is not known but probably genetic.
While this disease is a non_serious condition of Kidneys and does not cause very serious problems like renal failure ; however it can predispose to certain complications. These include
1.kidney stones,
2. Recurrent infections
3.pain in flanks
4. Rarely blood in urine or other serious problems.

So mostly it should not concern us a lot. But we need to take care to avoid the above complications. Or to treat if any complication occur like treat the UTi. We advise to take a lot of fluids to help avoid stones and infections. Rest there is no need to worry alot.
You may need to follow with your kidney doctor just to keep an eye to help if any complication may occur.

Hope I have answered your query. Please feel free to ask if you have any further questions or concerns. I will be glad to answer your questions.

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