Practice question

Mutant rescued only by final product demonstrates metabolic lesion immediately upstream of last conversion, so all intermediates earlier than block fail to bypass defect because they still require faulty enzyme for transformation. Only complete end product serves functional role in cellular metabolism and allows growth on minimal medium. Conversely mutants blocked earlier are rescued by multiple downstream intermediates plus final product showing broader rescue profile. Therefore restriction of rescue to final compound maps mutation to terminal step of biosynthetic sequence refining pathway order and identifying last enzyme uniquely.