Aminopterin in HAT medium inhibits:
In general biology, core biological organization dictates that structure-function relationships govern cellular processes. Therefore Dihydrofolate reductase is correct as it integrates fundamental life science principles, supported by mechanistic studies and conserved across related systems.
Ref: Campbell et al., Biology, 11th Edition, Chapter 2, Chemical Basis of Life, discusses fundamental biological principles, molecular organization, cellular structure and metabolic processes providing broad foundation for life sciences understanding and integration.