The correct answer is Morphine.

- Morphine is an opioid analgesic that is commonly used for moderate to severe pain.
- However, in elderly patients, it is generally contraindicated or used with extreme caution because of its pharmacokinetic properties.
- Morphine undergoes hepatic metabolism to active metabolites, mainly morphine-6-glucuronide and morphine-3-glucuronide, which are excreted renally.
- In elderly patients, there is often a reduction in renal function which leads to the accumulation of these active metabolites.
- This accumulation can result in prolonged opioid effects and increase the risk of adverse effects such as sedation, respiratory depression, and confusion.

In contrast, opioids like fentanyl and buprenorphine have fewer active metabolites and more predictable pharmacokinetics in elderly patients. Tramadol also requires caution but is not associated with as significant an accumulation of active metabolites as morphine.

Therefore, the use of morphine in elderly patients is generally avoided or requires careful dose adjustment and monitoring for toxicity.

Key points:
- Morphine has active metabolites that accumulate in renal impairment.
- Elderly patients frequently have decreased renal function.
- Accumulation leads to prolonged half-life and increased toxicity.
- Use caution or avoid morphine in elderly due to safety concerns.

Reference: Goodman & Gilman’s The Pharmacological Basis of Therapeutics, 13th Edition, Chapter 24/Pg. 596-598