**Core Concept**
Enzymes are biological molecules, typically proteins, that catalyze specific chemical reactions in the body. They are crucial for various physiological processes, including metabolism, DNA replication, and signal transduction. In this context, we are testing the student's understanding of the definition and characteristics of enzymes.

**Why the Correct Answer is Right**
Enzymes are proteins that consist of one or more polypeptide chains, which are sequences of amino acids linked by peptide bonds. These proteins have a unique shape that allows them to bind to specific substrates and facilitate chemical reactions. The correct answer is a type of enzyme that does not fit this definition, which is a nucleic acid enzyme.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is a type of protein, specifically a proteolytic enzyme that breaks down proteins into smaller peptides or amino acids.
**Option B:** This option is incorrect because it is a type of protein, specifically a phosphodiesterase enzyme that regulates the levels of cyclic nucleotides in the body.
**Option C:** This option is incorrect because it is a type of protein, specifically a deoxyribonuclease enzyme that breaks down DNA into smaller fragments.

**Clinical Pearl / High-Yield Fact**
A key concept to remember is that enzymes are highly specific, with each enzyme catalyzing a unique chemical reaction. This specificity is due to the unique shape and chemical properties of the enzyme, which allows it to bind to specific substrates.

**Correct Answer:** D. RNA (Ribozymes are RNA molecules that have enzymatic activity, but they are not proteins.)