Each of the following is a dentine-pulp response to slowly advancing caries EXCEPT one. Which is the EXCEPTION?

Correct answer: C. A rise in immunoglobulin concentration within the affected dentine

• A. DEAD TRACTS are genuine. They are dentinal tubules whose odontoblast processes have died, leaving them empty and filled with air. They appear BLACK in transmitted light and white in reflected light, and because they remain patent they transmit stimuli readily — which is one reason a tooth can be sensitive out of proportion to the lesion size.
• B. TERTIARY DENTINE is genuine, laid down at the pulpal surface directly beneath the lesion — REACTIONARY dentine by surviving odontoblasts responding to a mild stimulus, REPARATIVE dentine by newly differentiated odontoblast-like cells after a stronger one has killed the originals.
• C. CORRECT as the exception. Salivary immunoglobulin — secretory IgA — is produced by plasma cells in the salivary glands and acts in the SALIVA, agglutinating bacteria in the fluid phase. It is not a dentine response, it does not accumulate within the tooth as the lesion advances, and caries does not cause it to rise in the affected dentine. The other three are all genuine structural responses of the dentine-pulp complex.
• D. SCLEROTIC or TRANSLUCENT dentine is genuine and is the tooth's first line of defence. Peritubular dentine is laid down within the tubule lumen until it occludes, reducing permeability and slowing the ingress of acid and bacteria. Its presence is why a slowly advancing lesion is far less symptomatic than a rapid one.

Note: Picture the zones from the outside inward: necrotic and infected dentine, then demineralised AFFECTED dentine, then the sclerotic (translucent) zone, then dead tracts where processes have died, then tertiary dentine at the pulpal wall. Only AFFECTED dentine remineralises and may be left; INFECTED dentine must be removed. This is the basis of selective caries removal.