E.

Choice A rationale:

Trismus, also known as lockjaw, is a hallmark sign of tetanus. It is characterized by the inability to open the mouth due to sustained contraction of the jaw muscles.

Choice B rationale:

Dyspnea, or difficulty breathing, is a possible symptom of tetanus. Tetanus can lead to muscle spasms, including those of the respiratory muscles, which can result in respiratory distress and difficulty breathing.

Choice C rationale:

Ptosis, or drooping eyelid, is not a characteristic sign of tetanus. It is not directly associated with the muscle spasms and rigidity seen in tetanus.

Choice D rationale:

Tachycardia, or rapid heart rate, is not typically associated with tetanus. Tetanus primarily affects the skeletal muscles and does not directly affect the heart rate.

Choice E rationale:

Risus sardonicus, a facial muscle spasm resulting in a sardonic grin-like expression, is a classic sign of tetanus and is caused by sustained contraction of facial muscles.