Final answer:
The dramatic fact learned about Tom Robinson is that he has a disabled left arm, which challenges the credibility of Mayella Ewell's accusations against him.
Step-by-step explanation:
In To Kill a Mockingbird, the dramatic fact we learn about Tom Robinson at the end of Mayella's testimony is that Tom has a disabled left arm. This is a crucial detail because it directly contradicts Mayella's accusation that Tom had beaten and raped her, suggesting that such an assault would be difficult, if not impossible, with his impaired arm. Throughout the trial, Tom's physical limitation becomes a significant point that Atticus Finch uses to cast doubt on the Ewell's allegations.