**Core Concept**
During starvation, the body undergoes metabolic adaptations to conserve glucose and produce energy from alternative sources. This involves the upregulation of key enzymes involved in gluconeogenesis and the downregulation of those involved in glycolysis.

**Why the Correct Answer is Right**
Pyruvate kinase is a crucial enzyme in the glycolytic pathway, catalyzing the transfer of a phosphate group from phosphoenolpyruvate (PEP) to ADP, producing one molecule of pyruvate and one molecule of ATP. In starvation, the body seeks to conserve glucose and instead produce glucose through gluconeogenesis. As a result, the activity of pyruvate kinase is decreased, reducing glucose production and redirecting glucose precursors towards gluconeogenesis. This is in contrast to enzymes involved in gluconeogenesis, such as pyruvate carboxylase, PEP carboxykinase, and glucose-6-phosphatase, whose activities are increased to facilitate glucose production.

**Why Each Wrong Option is Incorrect**
**Option A:** Pyruvate carboxylase is an enzyme involved in gluconeogenesis, catalyzing the carboxylation of pyruvate to form oxaloacetate. Its increased activity in starvation is essential for the production of glucose.
**Option C:** PEP carboxykinase is another key enzyme in gluconeogenesis, catalyzing the conversion of oxaloacetate to phosphoenolpyruvate (PEP). Its increased activity in starvation helps to produce glucose.
**Option D:** Glucose-6-phosphatase is the final enzyme in the gluconeogenic pathway, catalyzing the conversion of glucose-6-phosphate to glucose. Its increased activity in starvation is crucial for glucose production and release into the bloodstream.

**Clinical Pearl / High-Yield Fact**
In starvation, the body's metabolic adaptations can be summarized by the phrase "gluconeogenesis wins over glycolysis." This highlights the shift in metabolic priorities from glucose utilization to glucose production, which is achieved through the upregulation of gluconeogenic enzymes and the downregulation of glycolytic enzymes like pyruvate kinase.

**✓ Correct Answer: B. Pyruvate kinase**