**Core Concept**
Spontaneous non-enzymatic degradation refers to the breakdown of a drug molecule in plasma without the involvement of enzymes. This type of degradation is often temperature-dependent and can occur due to the inherent chemical properties of the drug.

**Why the Correct Answer is Right**
The correct answer is **A. Oxytetracycline**. Oxytetracycline is a broad-spectrum antibiotic that undergoes spontaneous non-enzymatic degradation in plasma. This process is accelerated by heat, light, and alkaline pH, leading to the formation of degradation products. The structural instability of oxytetracycline makes it prone to non-enzymatic degradation, which can affect its efficacy and safety.

**Why Each Wrong Option is Incorrect**
* **Option B:** **Ampicillin** is typically inactivated by beta-lactamase enzymes, not spontaneous non-enzymatic degradation.
* **Option C:** **Phenytoin** is metabolized by the liver through the cytochrome P450 enzyme system, not spontaneous non-enzymatic degradation.
* **Option D:** **Warfarin** is metabolized by the liver through a combination of oxidative and reductive reactions, not spontaneous non-enzymatic degradation.

**Clinical Pearl / High-Yield Fact**
Spontaneous non-enzymatic degradation can be a significant factor in the stability and efficacy of certain drugs. Understanding the chemical properties of a drug and its susceptibility to degradation can inform dosing regimens and storage conditions.

**Correct Answer:** A. Oxytetracycline