The key condition that must exist in a cell for fermentation to occur is the absence of oxygen.

Here's why:

* Fermentation is an anaerobic process: This means it doesn't require oxygen to happen.

* Cellular respiration normally uses oxygen: The main way cells produce energy (ATP) is through aerobic respiration, which requires oxygen.

* Fermentation as a backup: When oxygen is scarce or unavailable, cells switch to fermentation as an alternative way to generate a small amount of ATP. This process is less efficient than aerobic respiration, but it allows the cell to survive in oxygen-deprived environments.

In summary, fermentation is a metabolic "backup" that cells use when they can't access oxygen.