**Core Concept**
The maximum plasma protein binding of a drug refers to the highest percentage of a drug that binds to plasma proteins, such as albumin and alpha-1 acid glycoprotein, in the bloodstream. This binding influences the drug's distribution, elimination, and potential toxicity.

**Why the Correct Answer is Right**
Warfarin is a highly protein-bound drug, with approximately 99% of it bound to plasma proteins, primarily albumin. This high degree of protein binding reduces the free fraction of the drug available for distribution to tissues and elimination by the kidneys. As a result, warfarin's pharmacokinetics and pharmacodynamics are significantly influenced by its protein binding.

**Why Each Wrong Option is Incorrect**
**Option A:** Lidocaine has a high protein binding, but it is significantly lower than warfarin, typically around 70-80%.
**Option B:** Digoxin has moderate protein binding, approximately 25%, which is not as high as warfarin.
**Option C:** Phenytoin has high protein binding, but it is generally lower than warfarin, typically around 90%.

**Clinical Pearl / High-Yield Fact**
Warfarin's high protein binding is a critical factor in its pharmacokinetics, making it essential to consider when monitoring patients on this medication, particularly in cases of renal impairment or liver disease.

**Correct Answer: C. Phenytoin**