**Core Concept**
Pyruvate kinase is a crucial enzyme in the glycolytic pathway, catalyzing the conversion of phosphoenolpyruvate (PEP) to pyruvate, generating one ATP molecule in the process. In the absence of pyruvate kinase, this step is blocked, affecting the overall energy yield from glucose metabolism.

**Why the Correct Answer is Right**
In the presence of pyruvate kinase, the conversion of 1 mole of glucose through glycolysis yields a net gain of 2 ATP molecules. However, in an embryo with a complete deficiency of pyruvate kinase, the conversion of phosphoenolpyruvate (PEP) to pyruvate is blocked. This results in the accumulation of PEP, which can then be converted to oxaloacetate via phosphoenolpyruvate carboxykinase, bypassing the need for pyruvate kinase. This bypass generates a net gain of 1 ATP molecule (from the conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate) instead of 2. Therefore, the net gain of ATP from the conversion of 1 mole of glucose is 1 mole.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not take into account the bypass mechanism that occurs in the absence of pyruvate kinase.
* **Option B:** This option is incorrect because it implies a complete block in glycolysis, which is not the case in a pyruvate kinase deficiency.
* **Option C:** This option is incorrect because it suggests a net loss of ATP, which is not consistent with the bypass mechanism that occurs in the absence of pyruvate kinase.

**Clinical Pearl / High-Yield Fact**
In the context of pyruvate kinase deficiency, it is essential to recognize that the enzyme's absence leads to a bypass mechanism that affects the overall energy yield from glucose metabolism. This knowledge is crucial for understanding the pathophysiology of this condition and its clinical implications.

**Correct Answer:** A.