**Core Concept**
Functional plasma enzymes are those that are involved in the body's metabolic processes and can be measured in the plasma to assess organ function, whereas non-functional plasma enzymes are not directly involved in metabolism but can be elevated in response to tissue damage.

**Why the Correct Answer is Right**
Pyruvate kinase is an example of a functional plasma enzyme because it plays a crucial role in glycolysis, the process by which glucose is converted into pyruvate to produce energy. It is a key enzyme in the glycolytic pathway, catalyzing the transfer of a phosphate group from phosphoenolpyruvate to adenosine diphosphate (ADP) to form pyruvate and ATP. Elevated levels of pyruvate kinase can indicate increased glycolytic activity, which can be seen in conditions such as cancer or infections.

**Why Each Wrong Option is Incorrect**
* **Option A:** Creatine kinase is a non-functional plasma enzyme because it is not directly involved in metabolic processes. Instead, it is a marker of muscle damage and is elevated in response to muscle injury or disease.
* **Option B:** Alkaline phosphatase is a non-functional plasma enzyme that is involved in the removal of phosphate groups from molecules, but it is not directly involved in metabolic processes. Elevated levels of alkaline phosphatase can indicate bone or liver disease.
* **Option D:** Lactate dehydrogenase is a non-functional plasma enzyme that is involved in the conversion of lactate to pyruvate, but it is not a key enzyme in the glycolytic pathway. Elevated levels of lactate dehydrogenase can indicate tissue damage or disease.

**Clinical Pearl / High-Yield Fact**
When assessing organ function, it's essential to distinguish between functional and non-functional plasma enzymes. Functional enzymes like pyruvate kinase provide valuable insights into metabolic processes, whereas non-functional enzymes like creatine kinase indicate tissue damage.

**Correct Answer:** C. Pyruvate kinase.