**Core Concept**
Tardive dyskinesia is a side effect caused by long-term use of antipsychotic medications, characterized by involuntary, repetitive movements of the face and limbs. This condition is often associated with the use of first-generation antipsychotics, such as trifluphenazine.

**Why the Correct Answer is Right**
The patient's symptoms of 'stiffness and twisting' of his neck and jaw, along with his medication history of lithium and trifluphenazine, strongly suggest the development of tardive dyskinesia. The next immediate step in management is to discontinue the offending agent, trifluphenazine, to prevent further exacerbation of the condition. This is because the pathophysiology of tardive dyskinesia involves long-term blockade of dopamine receptors in the basal ganglia, leading to abnormal movement patterns.

**Why Each Wrong Option is Incorrect**
**Option A:** Administering a benzodiazepine would not address the underlying cause of tardive dyskinesia and may even worsen the condition by sedating the patient further.
**Option B:** Increasing the dose of lithium is not relevant to the management of tardive dyskinesia and may lead to lithium toxicity.
**Option C:** Ordering a CT scan would not provide any immediate benefit in managing the patient's symptoms and would only serve to delay the discontinuation of trifluphenazine.

**Clinical Pearl / High-Yield Fact**
Tardive dyskinesia is often irreversible, so prompt recognition and discontinuation of the offending agent are crucial to prevent long-term damage.

**Correct Answer:** C. Discontinue the offending agent (trifluphenazine).