A 17 year old woman notices that her urine becomes red after she is given sulfonamides for treatment of a urinary tract infection. Both urine and serum test positive for free hemoglobin, and the urine red cell count is 1.2 million/mm3. A peripheral blood smear showed normocytic and normochromic red cells and a few “bite cells.” Deficiency of which of the following substances is most likely responsible for these symptoms?

**Core Concept**
Glucose-6-phosphate dehydrogenase (G6PD) deficiency is a genetic disorder that affects the enzyme responsible for protecting red blood cells from oxidative damage. Sulfonamides and other medications can trigger hemolysis (red blood cell breakdown) in individuals with this deficiency.

**Why the Correct Answer is Right**
In G6PD deficiency, the enzyme is lacking or deficient, leading to an accumulation of superoxides that damage the red blood cell membrane. Sulfonamides can induce oxidative stress, which triggers the breakdown of red blood cells, resulting in hemoglobinuria (red urine) and hemolytic anemia. The presence of "bite cells" on the peripheral blood smear is a characteristic feature of G6PD deficiency, indicating that the red blood cells have been subjected to oxidative damage.

**Why Each Wrong Option is Incorrect**
* **Option A:** Pyruvate kinase deficiency is a different enzyme deficiency that affects the glycolytic pathway, leading to a different type of hemolytic anemia. It is not associated with the specific symptoms described in this scenario.
* **Option B:** Folate deficiency can cause megaloblastic anemia, which is characterized by large, immature red blood cells. It is not related to the hemolysis and oxidative damage seen in G6PD deficiency.
* **Option C:** Vitamin B12 deficiency can also cause megaloblastic anemia, similar to folate deficiency. It is not associated with the specific symptoms described in this scenario.

**Clinical Pearl / High-Yield Fact**
G6PD deficiency is a common genetic disorder that affects millions of people worldwide, particularly in regions with high frequencies of malaria. It is essential to recognize this condition and avoid triggering medications that can lead to hemolysis.

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

✓ Correct Answer: C. Glucose-6-phosphate dehydrogenase