The metabolic pathway common to both cellular respiration and fermentation is glycolysis.

Here's why:

* Glycolysis is the initial breakdown of glucose into pyruvate. This process occurs in the cytoplasm of the cell and generates a small amount of ATP (energy) and NADH (electron carrier).

* Cellular respiration uses the pyruvate produced by glycolysis to further generate ATP through the Krebs cycle and electron transport chain. It requires oxygen.

* Fermentation takes place when oxygen is not available. It uses the pyruvate from glycolysis to regenerate NAD+ (another electron carrier needed for glycolysis to continue) and produce other byproducts like lactic acid or ethanol.

Therefore, both cellular respiration and fermentation rely on glycolysis as the starting point for their respective metabolic pathways.