Which substance is known to form crystals and is less toxic than ammonia and urea?

Uric acid is a byproduct of purine metabolism and is known for its lower toxicity compared to ammonia and urea. When the body metabolizes purines, it converts them into uric acid, which is less soluble in water than urea, allowing for the efficient excretion of nitrogenous waste in organisms such as birds and reptiles that need to conserve water. Uric acid crystallizes easily and is excreted as a paste or solid, which minimizes water loss in these animals.

In contrast, ammonia is highly toxic and needs to be diluted in water to be safely excreted, which is why it is primarily dealt with in aquatic organisms. Urea is less toxic than ammonia but still requires considerable water for excretion, making uric acid a more efficient form of nitrogen waste disposal in arid environments. Other options like creatinine, cholesterol, and glycogen do not serve the same purpose as waste products and are either more toxic or irrelevant to the topic of nitrogenous waste excretion.