A 3-year-old boy was found to have reduced red blood cell (RBC) numbers yet exhibited very few signs of anemia. An analysis of labeled RBCs indicated a greatly reduced ATP yield as compared to someone without the anemia. In this child, which one of the following would be expected to increase in RBC?

**Core Concept**
In this scenario, the child has a disorder affecting the production of ATP in red blood cells, leading to a reduced ATP yield. This condition is characterized by a defect in the glycolytic pathway, specifically the step involving the enzyme glyceraldehyde-3-phosphate dehydrogenase.

**Why the Correct Answer is Right**
The reduced ATP yield in the child's RBCs indicates a problem with energy production. In this case, the enzyme glyceraldehyde-3-phosphate dehydrogenase is impaired, leading to a decrease in the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate. This results in a decrease in the production of NADH and FADH2, which are essential for the electron transport chain and ATP production. To compensate for the reduced ATP yield, the RBCs would upregulate the enzyme pyruvate kinase to increase the conversion of phosphoenolpyruvate to pyruvate, thereby bypassing the impaired step and maintaining energy production.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the child's condition does not involve a defect in the electron transport chain or the production of NADH and FADH2.
* **Option B:** This option is incorrect because the child's condition does not involve an increase in 2,3-bisphosphoglycerate, which is a byproduct of glycolysis in RBCs.
* **Option C:** This option is incorrect because the child's condition does not involve an increase in the enzyme phosphofructokinase, which is involved in a different step of glycolysis.

**Clinical Pearl / High-Yield Fact**
In disorders affecting energy production in RBCs, the cells often upregulate enzymes involved in energy production to compensate for the impaired step. This can lead to an increase in the activity of certain enzymes, which can be detected through biochemical assays.

**Correct Answer:** C.

✓ Correct Answer: D. The levels of 2,3-bisphosphoglycerate