Hot tooth is:: (A) Tooth in line of fracture, (B) Difficult to anesthetise, (C) Vertical line fracture, (D) Very sensitive to heat test
Answer is B. A tooth that is difficult to anesthetize is known as a “hot tooth”. 
This is most commonly encountered in a mandibular first molar tooth wherein after the anesthetic block, the patient may describe profound numbness of the ipsilateral lip and tongue but still may experience acute pain during the access opening procedure.

Ref: Grossman’s endodontics, 13th edition, pg no:149