Final answer:
Hexagonal uric acid crystals can be distinguished from cystine crystals based on their solubility in hydrochloric acid; uric acid is soluble, but cystine is not.
Step-by-step explanation:
Hexagonal uric acid crystals can be distinguished from cystine crystals by using various tests. In the context of urine analysis, which is often where these types of crystals are encountered due to conditions like gout and specific types of kidney stones, a clear distinction can be made based on solubility and chemical reactivity. One method is to consider that cystine is insoluble in hydrochloric acid, but uric acid is soluble.
This property allows for differentiation by treating the respective crystals with hydrochloric acid and observing which ones dissolve and which remain intact. The positive nitroprusside test after reduction with sodium cyanide is another distinguishing feature exclusive to cystine. In contrast, pigmentation is not a reliable distinguishing feature because cystine crystals vary in color but are often colorless, while uric acid crystals can appear pigmented or clear.
Finally, the pH at which each type of crystal forms can also serve as a clue, with cystine forming at neutral or alkaline pH and uric acid forming at neutral to acidic pH. Considering these points, the correct way to distinguish hexagonal uric acid crystals from cystine crystals is option A: Cystine is insoluble in hydrochloric acid but uric acid is soluble.