The organ that secretes enzymes that are active at a low pH is the stomach.

Here's why:

* Low pH: The stomach's environment is highly acidic, with a pH ranging from 1.5 to 3.5. This acidity is crucial for the proper functioning of the stomach's digestive enzymes.

* Pepsin: The primary enzyme secreted by the stomach is pepsin. Pepsin is a protease (protein-digesting enzyme) that works best in acidic conditions.

* Other enzymes: The stomach also secretes other enzymes like gastric lipase, which breaks down fats, but these are less important than pepsin.

So, the combination of a low pH environment and the presence of pepsin makes the stomach a key player in protein digestion.