Which enzyme is responsible for curdling milk?

The enzyme responsible for curdling milk is rennin, also known as chymosin. This enzyme plays an essential role in the digestion of milk in young mammals. It works by coagulating milk proteins, specifically casein, causing the milk to form a gel-like consistency or curds. This process is crucial in the dairy industry for cheese production, as the curds can be separated from the whey and processed further to create various types of cheese.

In contrast, pepsin is primarily involved in breaking down proteins in the stomach, lactase is responsible for the digestion of lactose, the sugar present in milk, and amylase is involved in the breakdown of starches into sugars. While these enzymes have significant roles in digestion and food processing, they do not curdle milk like rennin does. Thus, the specific action of rennin in milk curdling is what makes it the correct answer.