Donna. Of course they would show him Donna. He couldn't feel guilty for Martha or Rose, not really. They got lives they wanted, but Donna. Oh, Donna lost everything. Everything because he had no choice but to decide for her.

No time to dwell. He pulled out the sonic and fiddled with a door lock. It popped open easily, revealing another corridor.

The Doctor inhaled, but smelled nothing.

"Probably some sort of mineral deposits in the walls," he theorized. "Might be mucking about with your senses, try not to think about it.