**Core Concept**
Abzyme, a term coined by combining "antibody" and "enzyme," refers to enzymes that are antibodies that exhibit catalytic activity. These unique molecules possess both immunoglobulin structure and enzyme function, enabling them to catalyze chemical reactions.

**Why the Correct Answer is Right**
Abzymes are engineered or naturally occurring antibodies that have been modified to possess enzymatic activity. This is typically achieved through the introduction of an active site, similar to that found in enzymes, which enables the antibody to catalyze specific chemical reactions. The catalytic activity of abzymes is often directed towards a specific substrate, and they can exhibit high specificity and efficiency in their reactions.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect, as this option does not accurately describe abzymes. While some antibodies may exhibit enzymatic activity, the term "abzyme" specifically refers to antibodies that have been engineered or modified to possess catalytic activity.

**Option B:** Incorrect, as this option describes a different type of molecule altogether. Enzymes are a broad class of molecules that exhibit catalytic activity, but not all enzymes are antibodies.

**Option C:** Incorrect, as this option does not accurately describe abzymes. While some antibodies may be used as therapeutic agents, the term "abzyme" specifically refers to antibodies that have been engineered or modified to possess catalytic activity.

**Clinical Pearl / High-Yield Fact**
Abzymes have potential applications in the development of novel therapeutic agents, such as enzyme-based drugs or diagnostic tools. Their unique ability to catalyze specific chemical reactions makes them attractive candidates for a range of biomedical applications.

**Correct Answer: D. Enzyme-antibody hybrid**