**Core Concept**
Mitochondrial myopathies are a group of disorders characterized by dysfunction of the mitochondria, leading to muscle weakness and other symptoms. Certain medications can induce mitochondrial myopathies, affecting the muscle's ability to produce energy.

**Why the Correct Answer is Right**
Zidovudine, a nucleoside reverse transcriptase inhibitor (NRTI), is an antiretroviral medication used to treat HIV/AIDS. It is known to cause a myopathy resembling mitochondrial myopathy with ragged red fibers. This condition is due to the drug's inhibition of mitochondrial DNA polymerase gamma, leading to mitochondrial dysfunction and muscle damage. The resulting muscle biopsy shows ragged red fibers, a hallmark of mitochondrial myopathies.

**Why Each Wrong Option is Incorrect**
* **Option A:** Stavudine, another NRTI, can also cause mitochondrial toxicity, but it is not the primary medication associated with myopathy resembling mitochondrial myopathy with ragged red fibers.
* **Option B:** Lamivudine, a nucleoside reverse transcriptase inhibitor, is generally well-tolerated and not commonly associated with mitochondrial toxicity.
* **Option C:** Efavirenz, a non-nucleoside reverse transcriptase inhibitor, is not typically associated with mitochondrial myopathy.

**Clinical Pearl / High-Yield Fact**
When prescribing antiretroviral therapy, it is essential to be aware of the potential for mitochondrial toxicity, particularly with NRTIs like zidovudine. Regular monitoring of muscle function and liver enzymes is crucial to prevent and detect adverse effects early.

**Correct Answer:** A. Zidovudine.