**Core Concept**
G6PD stands for Glucose-6-phosphate dehydrogenase, a crucial enzyme in the pentose phosphate pathway that protects red blood cells from oxidative damage.

**Why the Correct Answer is Right**
Glucose-6-phosphate dehydrogenase is an enzyme that catalyzes the conversion of glucose-6-phosphate to 6-phosphogluconolactone, generating NADPH in the process. NADPH is essential for maintaining the reduced state of glutathione, which in turn protects red blood cells from oxidative damage caused by hydrogen peroxide and other reactive oxygen species. Deficiencies in G6PD lead to hemolytic anemia, particularly in response to oxidative stressors such as certain medications, infections, and foods.

**Why Each Wrong Option is Incorrect**
**Option A:** Glucose-6-phosphate isozyme does not accurately describe the enzyme's function or role in the pentose phosphate pathway.
**Option B:** Glucose-6-phosphate dehydrogenase deficiency is a correct association, but it is not the full name of the enzyme.
**Option C:** Glucose-6-phosphate kinase is a different enzyme in the glycolytic pathway, unrelated to the pentose phosphate pathway.

**Clinical Pearl / High-Yield Fact**
G6PD deficiency is a common genetic disorder affecting millions worldwide, particularly in populations from the Mediterranean, Africa, and Southeast Asia.

**Correct Answer:** D. Glucose-6-phosphate dehydrogenase