B. Magnesium ammonium phosphate – Explanation
Stag-horn calculi
- composed of Struvite (ammonium magnesium phosphate, triple phosphate)
- form in alkaline urine (ammonia producing bacteria such as Ureaplasma urealyticum and Proteus therefore
predispose)
Renal stones: imaging
The table below summarises the appearance of different types of renal stone on x-ray
| Type | Frequency | Radiograph appearance |
| Calcium oxalate | 40% | Opaque |
| Mixed calcium oxalate/phosphate stones | 25% | Opaque |
| Triple phosphate stones* | 10% | Opaque |
| Calcium phosphate | 10% | Opaque |
| Urate stones | 5-10% | Radio-lucent |
| Cystine stones | 1% | Semi-opaque, ‘ground-glass’ appearance |
| Xanthine stones | <1% | Radio-lucent |
*stag-horn calculi involve the renal pelvis and extend into at least 2 calyces. They develop in alkaline
urine and are composed of struvite (ammonium magnesium phosphate, triple phosphate).
Ureaplasma urealyticum and Proteus infections predispose to their formation
