Tyrosine crystals are fine, silky needles found in the urine in severe liver failure or a congenital disorder of amino acid metabolism, such as tyrosinemia. These crystals are colorless to brown and tend to clump together in bundles or rosette-like structures. They become strongly birefringent under polarized light and are often found alongside leucine crystals in patients with severe liver failure. Drug-induced crystals, such as from ampicillin or acyclovir, should also be considered in the differential diagnosis.