**Core Concept**
Opioids are a class of medications used to treat pain, but they can have various side effects, including cardiovascular effects. Tachycardia, or an increased heart rate, is an abnormal response to certain opioids.

**Why the Correct Answer is Right**
Fentanyl is a synthetic opioid with a rapid onset and short duration of action. It is a potent agonist at the mu-opioid receptor, which can lead to bradycardia (decreased heart rate) in most cases. However, fentanyl can cause tachycardia, particularly in patients with hypovolemia or those who are already tachycardic. This is because fentanyl can cause vasodilation and decreased peripheral resistance, leading to a reflex tachycardia.

**Why Each Wrong Option is Incorrect**
**Option A:** Morphine is a natural opioid that primarily acts on the mu-opioid receptor, leading to bradycardia and decreased cardiac output. It is not associated with tachycardia.
**Option B:** Methadone is a long-acting opioid that can cause QT prolongation and cardiac arrhythmias, but it is not typically associated with tachycardia.
**Option C:** Buprenorphine is a partial agonist at the mu-opioid receptor and is not typically associated with tachycardia.

**Clinical Pearl / High-Yield Fact**
Fentanyl is a highly lipophilic opioid that can cause rapid onset of effects, but it can also lead to unexpected cardiovascular side effects, such as tachycardia, particularly in patients with hypovolemia or underlying cardiovascular disease.

**Correct Answer:** B.