**Core Concept**
Rennin, also known as chymosin, is a gastric enzyme that plays a crucial role in the digestion of milk proteins. It is produced in the stomach and works in conjunction with gastric amylase to break down casein, a key component of milk.

**Why the Correct Answer is Right**
Rennin is secreted by the chief cells in the stomach lining and is activated by pepsin, another gastric enzyme. Once activated, rennin cleaves the phosphopeptide bonds in casein, denaturing the protein and making it more accessible to other digestive enzymes. This process is essential for the proper digestion and absorption of milk proteins in infants and young animals.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because rennin is not produced by the pancreas, which secretes a wide range of digestive enzymes, but is specifically produced in the stomach lining.
**Option B:** This option is incorrect because gastric lipase, not rennin, is responsible for the breakdown of triglycerides in the stomach.
**Option C:** This option is incorrect because gastric amylase, not rennin, is responsible for the breakdown of carbohydrates in the stomach.

**Clinical Pearl / High-Yield Fact**
The rennin-angiotensin-aldosterone system (RAAS) plays a critical role in regulating blood pressure and fluid balance in the body. Understanding the relationship between rennin and blood pressure is essential for the diagnosis and management of hypertension.

**Correct Answer:** D.