## Core Concept
Covalent modification is a regulatory mechanism for enzyme activity that involves the addition or removal of functional groups. This process can activate or deactivate enzymes, providing a rapid and reversible means of controlling metabolic pathways. Key examples include phosphorylation and dephosphorylation.

## Why the Correct Answer is Right
Glycogen phosphorylase is a well-known example of an enzyme regulated by covalent modification, specifically through phosphorylation. This process activates glycogen phosphorylase, allowing for the breakdown of glycogen into glucose-1-phosphate. On the other hand, enzymes like **phenylalanine hydroxylase** are primarily regulated by other mechanisms such as feedback inhibition or allosteric control rather than covalent modification.

## Why Each Wrong Option is Incorrect
* **Option A:** Glycogen phosphorylase is indeed regulated by covalent modification (phosphorylation), making it an incorrect choice for "all except."
* **Option B:** Phosphofructokinase-2 (PFK-2) is regulated by covalent modification. It is involved in the regulation of glycolysis and gluconeogenesis through the production of fructose-2,6-bisphosphate, and its activity is controlled by phosphorylation/dephosphorylation.
* **Option D:** Pyruvate dehydrogenase is another example of an enzyme regulated by covalent modification. It is controlled by phosphorylation/dephosphorylation, which determines its activity in the conversion of pyruvate to acetyl-CoA.

## Clinical Pearl / High-Yield Fact
A memorable point for exams is that covalent modification is a critical regulatory mechanism for many key enzymes in metabolism, especially those involved in energy production and storage. For instance, recall that glycogen synthase and glycogen phosphorylase are both regulated by phosphorylation status, which switches them between active and inactive states.

## Correct Answer Line
**Correct Answer: C. Phenylalanine hydroxylase**