Which of the following is a negative connotation regarding a substance?

The term "virulent" carries a negative connotation, primarily associated with harmfulness or toxicity. In scientific and medical contexts, it often describes a strain of a virus or bacteria that is particularly severe or capable of causing lethal disease. Beyond this specific usage, "virulent" can also depict anything that is intensely hostile or malicious, further emphasizing its negative implications.

In contrast, the other options offer more neutral or even positive associations. "Accomplished" suggests a high level of skill or achievement, "replete" indicates fullness or abundance in a positive manner, and "nascent" refers to something that is just beginning to develop, typically with a sense of potential or promise. Thus, "virulent" stands out as the only term with an inherently negative connotation concerning a substance, distinguishing it clearly from the others.