## Core Concept
Trypsin is a serine protease that plays a crucial role in the activation of various pancreatic enzymes. It is involved in the activation of several zymogens (inactive enzyme precursors) in the pancreas.

## Why the Correct Answer is Right
Trypsin activates several enzymes by cleaving their zymogen forms. For instance, it activates **chymotrypsinogen** to chymotrypsin, **procarboxypeptidase** to carboxypeptidase, and **prophospholipase A2**. However, trypsin itself is activated from trypsinogen by **enterokinase** (also known as enteropeptidase), an enzyme found in the intestinal brush border.

## Why Each Wrong Option is Incorrect
* **Option A:** Trypsin activates **chymotrypsinogen** to form chymotrypsin, which is crucial for protein digestion in the intestine.
* **Option B:** It also activates **procarboxypeptidase** to carboxypeptidase, which is involved in protein digestion.
* **Option C:** Trypsin activates **prophospholipase A2** to phospholipase A2, which plays a role in lipid digestion.

## Why Option D is Correct (and thus the Correct Answer)
* **Option D:** Trypsinogen is activated to trypsin by **enterokinase**, not by trypsin itself. This makes it the exception in the list as the question asks for an enzyme not activated by trypsin.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **enterokinase** (or enteropeptidase) is crucial for the activation of trypsinogen to trypsin. Deficiency in enterokinase leads to a condition where there's impaired activation of trypsin and subsequently other digestive enzymes, resulting in malabsorption.

## Correct Answer: D. trypsinogen.