The second in this series of stone forming phenotypes, the calcium phosphate stone formers are less numerous than the calcium oxalate stone formers, but perhaps more worrisome, and certainly more complex. There are two types, those whose stones contain any brushite – an unusual form of calcium phosphate in stones, and those whose phosphate is only hydroxyapatite – the mineral found in bones. This latter group is to a large extent composed of young women, for reasons we do not know. Phosphate stones are likely than the calcium oxalate variety to be numerous, and often produce nephrocalcinosis, a mixture of small stones and tissue calcium deposits. Nephrocalcinosis, in turn, is often labelled medullary sponge kidney simply on radiological grounds, even when the distinctive lesions of MSK are not necessarily present. Likewise, phosphate stone patients can appear to have renal tubular acidosis because of nephrocalcinosis and because RTA and phosphate stone patients both produce a more alkaline urine than do normals, or patients with calcium oxalate stones. All in all, this is a complex form of calcium stones, challenging for clinicians and often very trying and concerning for patients with it. The article is long and difficult, so you might want to watch this video by way of an introduction.
If you want a comprehensive view of what kidney stones are and how they are made, I have put together various of the articles in this by now rather large site to make up a kind of story, or narrative, or, as I like to think of it, a walking tour. Read in the order I suggest, and take a look at the few narrator comments and I promise a nice overview of the topic. More will come if people seem to like this format.