Final answer:
The antineoplastic agent used for ectopic pregnancy is methotrexate, which inhibits folic acid reductase and prevents DNA synthesis in the rapidly dividing cells of the ectopic tissue.
Step-by-step explanation:
The antineoplastic agent used in the treatment of ectopic pregnancy is methotrexate. Methotrexate inhibits the enzyme folic acid reductase necessary for the conversion of folic acid to its active form, tetrahydrofolic acid, which is essential in the synthesis of nucleotides used for DNA replication. By interrupting this process, methotrexate effectively stops the growth of rapidly dividing ectopic pregnancy cells.
Other antineoplastic agents like fluorouracil and nitrogen mustard have their respective mechanisms and applications in cancer treatment but are not the drugs of choice for ectopic pregnancies. Methotrexate's ability to target rapidly proliferating cells makes it suitable for this condition without the need for surgical intervention in many cases.