The following conditions were found in a mutant cell: Condition 1: The strength of the proton gradient in the mitochondria decreases over time Condition 2: ADP accumulates; ATP depleted over time Condition 3: NAD accumulates: NADH depletes over time Where does this mutant cell likely have a block/disruption

The correct answer is - citric acid cycle.

Explanation:

In the third condition, fresh NADH molecules are not forming as NAD+ is getting accumulated fresh NADH molecules are continuously released or generated from the citric acid cycle.

There is no NADH production occurs which means there is some problem with citric acid cyle. Other conditions given is also the result of this inhibition occurs in the citric acid cycle that are decrease in proton gradient and ATP depeletion.