Question 1 · Hard
A 34-year-old woman with Sjögren syndrome has passed two calcium-containing stones this year and takes no regular medicines. Serum: sodium 139 mmol/L, potassium 3.1 mmol/L, chloride 114 mmol/L, total CO2 15 mmol/L. A fresh fasting urine collected during this acidosis has pH 6.8 and SG 1.015, with negative nitrite and leukocyte esterase, and the culture shows no growth. The sediment contains many colorless, thin prisms, some arranged in rosettes, that dissolve in dilute acetic acid. No envelope-shaped or coffin-lid crystals are seen. Which crystal and underlying disorder fit best?
- ACalcium oxalate; hyperoxaluria from bowel disease
- BSulfonamide; crystals from antibiotic therapy
- CTriple phosphate; infection with a urease producer
- DCalcium phosphate; distal renal tubular acidosis
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Answer: D · Calcium phosphate; distal renal tubular acidosis
The serum results show a normal anion gap metabolic acidosis (139 - (114 + 15) = 10 mmol/L) with a low potassium. A urine pH of 6.8 during systemic acidosis means the distal nephron cannot secrete enough hydrogen ions, which defines distal (type 1) renal tubular acidosis, a known complication of Sjögren syndrome. The alkaline urine, high urine calcium and low urine citrate of distal RTA favor calcium phosphate stones and nephrocalcinosis. Colorless thin prisms and rosettes in neutral or alkaline urine that dissolve in dilute acetic acid are calcium phosphate crystals. In proximal RTA, by contrast, the urine can still be acidified once plasma bicarbonate falls, and stones are uncommon.
- A: Calcium oxalate forms envelopes or ovals and does not dissolve in dilute acetic acid.
- B: Sulfonamide rosettes are yellow-brown, form in acidic urine, and she takes no medicines.
- C: Triple phosphate forms coffin-lid prisms with a urease-producing infection; the culture is negative and no coffin lids are seen.