**Core Concept**
Zidovudine, a nucleoside reverse transcriptase inhibitor (NRTI), is a common antiretroviral medication used in the treatment of HIV infection. Its toxicity can be increased by various factors, including other medications that either inhibit its renal excretion or displace it from plasma protein binding sites.

**Why the Correct Answer is Right**
Azithromycin, a macrolide antibiotic, does not significantly interact with zidovudine's metabolism or renal excretion. Unlike other options, azithromycin does not have a notable effect on zidovudine's pharmacokinetics, making it a safe co-administration in clinical practice.

**Why Each Wrong Option is Incorrect**
**Option B:** Probencid is a uricosuric agent that inhibits the renal tubular secretion of organic acids, including zidovudine. By reducing its renal clearance, probencid increases zidovudine's plasma concentrations, thereby enhancing its toxicity.
**Option C:** Cisplatin is a platinum-based chemotherapeutic agent that can cause nephrotoxicity, which may impair zidovudine's renal excretion. This can lead to elevated zidovudine levels and increased toxicity.
**Option D:** Cyclophosphamide is an alkylating chemotherapeutic agent that can cause hemorrhagic cystitis, which may impair renal function. Reduced renal function can lead to decreased zidovudine clearance and increased toxicity.

**Clinical Pearl / High-Yield Fact**
When prescribing zidovudine, clinicians should be aware of potential interactions with other medications that may increase its toxicity. Regular monitoring of renal function and plasma zidovudine levels is crucial in patients receiving concurrent nephrotoxic or renal-excretion-inhibiting medications.

**✓ Correct Answer: A. Azithromycin**