**Core Concept**
Phosphofructokinase (PFK) is a key regulatory enzyme in glycolysis, catalyzing the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate. Fructose-2,6-bisphosphate (F-2,6-BP) is an allosteric regulator that modulates the activity of PFK and fructose-1,6-bisphosphatase, the enzyme responsible for the opposite reaction.

**Why the Correct Answer is Right**
Fructose-2,6-bisphosphate is a potent activator of PFK, increasing its activity by reducing the Km for fructose-6-phosphate and stabilizing the enzyme's active conformation. Conversely, F-2,6-BP inhibits fructose-1,6-bisphosphatase, thereby promoting the irreversible step of glycolysis. This dual regulation allows for fine-tuning of glycolytic flux in response to changing energy demands.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the regulation of PFK and fructose-1,6-bisphosphatase by fructose-2,6-bisphosphate.
**Option B:** This option is incorrect as it refers to a different regulatory mechanism involving AMP-activated protein kinase (AMPK), which is activated in response to energy stress.
**Option C:** This option is not directly related to the regulation of glycolysis by fructose-2,6-bisphosphate.

**Clinical Pearl / High-Yield Fact**
Fructose-2,6-bisphosphate is a key regulator of glycolysis, and its levels are tightly controlled by feedback mechanisms involving glucose, insulin, and glucagon. Understanding the role of F-2,6-BP is crucial for appreciating the complex interplay between glucose metabolism and energy homeostasis.

**Correct Answer:** C. Liver