**Core Concept:** Opioid withdrawal is a set of physiological responses that occur when an individual dependent on opioids tries to stop or reduce their usage abruptly or experiences a decrease in drug concentration. Opioids are a class of drugs that act on the body's opioid receptors, including morphine, heroin, and prescription painkillers like oxycodone and fentanyl.

**Why the Correct Answer is Right:**

The correct answer, C: Diarrhea, is a symptom of opioid withdrawal. Opioids stimulate the mu-opioid receptors in the gastrointestinal tract, leading to increased intestinal smooth muscle relaxation and increased secretion from the enteric glands, resulting in diarrhea. This is a result of the increased sensitivity to the GI tract stimulation due to opioid dependence and withdrawal.

**Why Each Wrong Option is Incorrect:**

A: Nausea and vomiting are not specific to opioid withdrawal but may be present in various other conditions.
B: Fatigue is a general symptom and not specific to opioid withdrawal.
D: Delirium tremens (DTs) is a severe complication of alcohol withdrawal and not related to opioid withdrawal.

**Clinical Pearl / High-Yield Fact:**

Understanding opioid withdrawal symptoms is crucial for healthcare professionals as it helps in diagnosing and managing patients with suspected opioid dependence. The severity and duration of withdrawal symptoms can guide the appropriate treatment plan, including the use of medications like methadone or buprenorphine for gradual tapering of opioid intake.

**Correct Answer:** C: Diarrhea.