Which of the following is a symptom of nerve agents?

The correct choice highlights tightness in the chest as a symptom associated with nerve agents. Nerve agents disrupt the normal functioning of the nervous system by inhibiting the enzyme acetylcholinesterase, which leads to an accumulation of acetylcholine at nerve synapses. This excessive buildup results in overstimulation of the muscles and respiratory system, ultimately causing symptoms such as tightness in the chest due to bronchoconstriction.

In contrast, while some symptoms can vary, increased heart rate is more commonly associated with a different category of agents. Rashes on the skin could arise from other irritants or exposures but are not characteristic of nerve agents specifically. Similarly, extreme fatigue is a more general symptom that can occur in various conditions and is not specifically indicative of nerve agent exposure. Thus, tightness in the chest distinctly relates to the physiological effects caused by nerve agents, making it the most appropriate answer.