**Core Concept**
G6PD (Glucose-6-phosphate dehydrogenase) deficiency is an X-linked recessive disorder characterized by a deficiency of the enzyme glucose-6-phosphate dehydrogenase. This enzyme plays a crucial role in protecting red blood cells from oxidative damage by maintaining the levels of reduced glutathione.

**Why the Correct Answer is Right**
The definitive finding of G6PD deficiency is the measurement of the enzyme activity in red blood cells. A deficiency of G6PD leads to an accumulation of oxidized glutathione, which in turn causes hemolysis. The enzyme activity is measured using a spectrophotometric assay, and a value below 10% of normal is considered diagnostic of G6PD deficiency. This measurement is used to confirm the diagnosis in individuals with a clinical presentation suggestive of G6PD deficiency, such as acute hemolytic anemia triggered by certain medications or infections.

**Why Each Wrong Option is Incorrect**
**Option A:** Elevated levels of glucose-6-phosphate dehydrogenase are not diagnostic of G6PD deficiency. In fact, the enzyme activity is decreased in affected individuals.
**Option B:** The presence of Heinz bodies is a clinical finding associated with G6PD deficiency, but it is not the definitive diagnostic finding.
**Option C:** The genetic mutation responsible for G6PD deficiency is an X-linked recessive mutation, but the measurement of enzyme activity is the definitive diagnostic test.

**Clinical Pearl / High-Yield Fact**
G6PD deficiency is a common genetic disorder affecting approximately 400 million people worldwide, making it one of the most prevalent genetic disorders globally.

**Correct Answer: B. The presence of Heinz bodies is a clinical finding associated with G6PD deficiency.**