**Core Concept**
PK reaction, also known as the Parke-Kawabata reaction, is a test used to detect the presence of a specific enzyme, pyruvate kinase, in red blood cells. Pyruvate kinase is an essential enzyme in the glycolytic pathway, catalyzing the conversion of phosphoenolpyruvate to pyruvate.

**Why the Correct Answer is Right**
The PK reaction detects the pyruvate kinase enzyme by measuring the rate of reduction of NADP+ to NADPH in the presence of phosphoenolpyruvate, ADP, and KCl. This enzymatic reaction is specific to pyruvate kinase and is used to diagnose pyruvate kinase deficiency, a rare genetic disorder characterized by chronic hemolytic anemia. The test is performed by mixing red blood cells with a reaction mixture containing the substrates and observing the increase in absorbance at 340 nm due to the reduction of NADP+.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not related to the PK reaction.
**Option B:** This option is incorrect because it refers to a different test, the glucose-6-phosphate dehydrogenase (G6PD) test, which detects a different enzyme.
**Option C:** This option is incorrect because it refers to a different reaction, the hexokinase reaction, which is not specific to pyruvate kinase.

**Clinical Pearl / High-Yield Fact**
The PK reaction is a specific test for pyruvate kinase deficiency, which is often diagnosed in newborns or young children with severe hemolytic anemia.

**Correct Answer: D.**