**Core Concept**
Opioid withdrawal is a complex physiological response to the abrupt cessation or significant reduction of opioid use. It involves the activation of various neurotransmitter systems, including the dopaminergic, serotonergic, and noradrenergic systems, which are normally suppressed by opioids. The withdrawal syndrome is characterized by a range of symptoms, including both physical and psychological manifestations.

**Why the Correct Answer is Right**
The correct answer is **yawning**. Yawning is a common symptom of opioid withdrawal, thought to be mediated by the activation of the brain's opioid receptors, particularly the mu-receptors. When opioids are withdrawn, there is a sudden release of dopamine, which can lead to abnormal yawning as a compensatory mechanism. This is in contrast to the normal physiological response to yawning, which is thought to be related to the regulation of body temperature and the activation of the brain's default mode network.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a recognized symptom of opioid withdrawal. While anxiety can be a symptom of withdrawal, it is not the most specific or distinctive symptom.
**Option B:** This option is incorrect because it is not a direct consequence of opioid withdrawal. While sleep disturbances can occur in withdrawal, they are not as directly related to the withdrawal process as yawning is.
**Option C:** This option is incorrect because it is not a recognized symptom of opioid withdrawal. While fatigue can be a symptom of withdrawal, it is not as specific or distinctive as yawning.

**Clinical Pearl / High-Yield Fact**
It's worth noting that opioid withdrawal can be treated with medications such as methadone, buprenorphine, and clonidine, which can help to mitigate the symptoms of withdrawal. Additionally, behavioral therapies such as cognitive-behavioral therapy can be effective in managing withdrawal symptoms and preventing relapse.

**Correct Answer:** C. Diaphoresis (sweating) is a common symptom of opioid withdrawal, but the best answer is not listed.