Final answer:
Failure to include pulp horns in a coronal access is the least likely option to cause tooth discoloration with a necrotic pulp, as it does not involve factors that lead to discoloration unlike the other options listed.
Step-by-step explanation:
The option least likely to cause discoloration of a tooth with a necrotic pulp is C. Failure to include pulp horns in a coronal access. This procedure is primarily concerned with the physical shape and access to the pulp chamber during the process of a root canal and does not inherently involve materials or factors that would discolor the tooth. In contrast, decomposition of necrotic tissue, hemolysis of unremoved blood cells, frequent ingestion of heavy doses of tetracycline, and use of a root canal sealer that contains precipitated silver can each lead to discoloration either through the breakdown of tissues or chemical interaction with the tooth structures.