Final answer:
The antagonist molecule that binds to and blocks plasma membrane receptors would not affect testosterone, as testosterone enters the cell and binds to intracellular receptors, not plasma membrane receptors.
Step-by-step explanation:
An antagonist molecule that binds to and blocks plasma membrane receptors will affect testosterone, a steroid hormone, by preventing it from binding to its receptors. Since steroid hormones like testosterone typically enter the cell and bind to intracellular receptors, they are not dependent on plasma membrane receptors. Therefore, the presence of this antagonist would not affect testosterone synthesis or its activity within the cell. It would not increase testosterone synthesis, decrease its synthesis, or convert testosterone to estrogen. Rather, the antagonist would impact peptide hormones that rely on receptors on the plasma membrane to initiate their signaling pathways, such as those involving cAMP.