Which of the following genetically engineered organisms was used by Eli Lilly to prepare human insulin?
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Recombinant DNA technology, which involves inserting a gene into a host organism like bacteria, is a common method for producing important proteins like insulin.
Eli Lilly used genetically engineered bacteria, specifically \textit{Escherichia coli}, to produce human insulin. This method of recombinant DNA technology involves inserting the human insulin gene into a bacterial plasmid, enabling the bacteria to produce the hormone. This was one of the first major applications of genetic engineering in medicine.
Thus, the correct answer is Bacterium.