Two adjacent mandibular premolars with MOD amalgams have an open/poorly formed proximal contact; the patient reports food impaction, pain, and gingival bleeding between them. The most likely cause is:

Correct answer: A. Poor contour of the matrix band (open/deficient contact)

• A. Correct. A poorly contoured matrix band (or improper wedging) creates an open contact or overhang, leading to food impaction, plaque, and interproximal bleeding/pain.
• B. Fluoride does not cause an open contact.
• C. A normal, tight contact would not cause food impaction or bleeding.
• D. The problem is contour/contact form, not the amount of tooth removed.