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Cystine = 2 cysteine molecules bound together by a disulfide bond.
Each cysteine has an –SH (thiol group), and oxidation makes them join together to form -S–S-. The same goes for homocysteine, where two homocysteines join to form homocystine.
In cystinuria, the PCT of the kidney cannot reabsorb cysteine, so the buildup of cysteine leads to the formation of hexagonal cystine stones in the renal tubules.
If the patient is young, there is likely an underlying hereditary pathology, so cystine stones should be high on your differential.
| A USMLE favorite is showing you a picture of hexagonal crystals in the urine. The diagnosis is cystinuria. |
The reabsorption of cysteine is not isolated. It is part of a larger malabsorptive pathology characterized by an inability to absorb cysteine, ornithine, lysine and arginine (COLA).
Therefore, on Step1, inability to absorb cysteine = inability to absorb ornithine, lysine and arginine.
| If a patient with Hx of cystinuria develops calcium phosphate stones and they ask you why, it’s because the citrate or HCO3– used as Tx increased the pH of the urine enough to allow for Ca3(PO4)2 stone formation. Furosemide is a wrong answer. Although loops cause hypercalciuria, they aren’t Tx for cystinuria. |
For cystinuria, either a picture of the hexagonal stone or a mention of it will be made ~50% of the time.
About ~15% of the time, it will rely on your knowing the COLA association.
Another ~15% of the time they will give a relatively vague vignette of non-specific nephrolithiasis in an otherwise healthy patient in his or her late-teens or 20s.
~20% of the time they’ll mention in a vignette that the patient has a (+) nitroprusside cyanide test, or they’ll say that the urine becomes purple/blue/magenta when nitroprusside and cyanide are added. This is a diagnostic test for both cystinuria and homocystinuria. The -S–S- becomes 2x –SH when reacted with cyanide, and then nitroprusside reacts with –SH to induce a color change.
If they mention cystine stones, they want you to know that alkalinizing the urine helps dissolve them. Citrate is commonly the answer when they ask for the treatment. Penicillamine rarely also shows up as an answer (helps cleave the -S–S- in cystine).
| Penicillamine is used as copper chelation therapy in Wilson disease. It also causes drug-induced SLE w/ (+) anti-histone Abs. For drug-induced SLE, remember Mom is HIPP: Minocycline, Hydralazine, INH, Procainamide, Penicillamine |
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