Calcium in urine combines with which of the following?

Calcium in urine primarily combines with sulfate ions to form calcium sulfate. This is significant because calcium sulfate is relatively soluble in urine, which helps regulate calcium levels in the body. The presence of sulfate in the urinary system allows for the excretion of excess calcium, maintaining mineral balance.

While proteins and nucleic acids can interact with calcium in various biochemical processes, they do not form prominent soluble complexes with calcium in urine in the same way that sulfates do. Thus, the primary association of calcium in the urinary context pertains to sulfates, validating the selection of this option as the correct answer.