In a Class II preparation, the isthmus is the portion of the cavity connecting an occlusal portion and proximal portion together.

• A. Class II
• B. Class I
• C. Both
• D. None

Answer: (A)

In cavity prep terminology, the isthmus is the narrow bridge that links the occlusal portion of a preparation to a proximal (box) portion. In a Class II preparation there is both an occlusal cavity and a proximal box, so the isthmus serves as the connecting region between these two parts. Class I preparations involve only pits and fissures on the occlusal surface and do not have a proximal box, so there isn’t an isthmus connecting occlusal and proximal areas. Therefore, the statement correctly identifies the isthmus as the portion that connects the occlusal and proximal areas in Class II.