What is a defining characteristic of a nerve agent?

A nerve agent is characterized primarily by its mechanism of action on the nervous system. Specifically, these agents inhibit the enzyme acetylcholinesterase, leading to an accumulation of the neurotransmitter acetylcholine at nerve synapses. This disruption causes continuous stimulation of muscles, glands, and the central nervous system, which can result in a range of severe symptoms and possibly fatal outcomes.

The incorrect options highlight aspects that do not define nerve agents. For instance, while endocrine disruption can occur with some toxic substances, it is not a characteristic feature of nerve agents, which are primarily neurotoxic. If nerve agents were primarily absorbed through the digestive system, it would limit their effectiveness, whereas they can be absorbed through various routes including inhalation, skin contact, and ingestion, making them highly versatile in their potential for poisoning. Additionally, nerve agents are not safe to touch; they can be absorbed through the skin and cause serious harm, underscoring the need for protective measures when dealing with such hazardous materials.