Final answer:
An antagonist molecule that blocks plasma membrane receptors will not significantly affect testosterone signaling because testosterone, a steroid hormone, enters cells directly and binds to intracellular receptors to mediate its effects.
Step-by-step explanation:
The effect of a newly discovered antagonist molecule that binds to and blocks plasma membrane receptors on testosterone, which is a steroid hormone, will be minimal or non-existent in terms of the hormone's ability to mediate signaling inside the target cell.
Steroid hormones like testosterone are hydrophobic molecules that can diffuse directly across the plasma membrane without the need for surface membrane receptors. After separating from carrier proteins in the blood, they bind to intracellular receptors in the cytoplasm or nucleus of target cells.
When testosterone binds to these intracellular receptors, the hormone-receptor complex can act as a transcription regulator in the nucleus, affecting gene expression by increasing or decreasing the synthesis of mRNA and, consequently, protein synthesis.
Therefore, since the antagonist molecule blocks only plasma membrane receptors, and testosterone does not rely on these receptors for cell entry or signaling, there will be no significant effect on testosterone-mediated signaling. The correct answer would be 'd. It will not affect testosterone-mediated signaling.'