**Core Concept**: Uveitis is an inflammation of the uveal tract (iris, ciliary body, and choroid) in the eye. Non-infectious uveitis refers to inflammation caused by autoimmune, allergic, or other non-infectious factors. Drug response trials are used to evaluate the effectiveness of a particular medication in treating specific conditions.

**Why the Correct Answer is Right**: The correct answer is **Option C: Clinical Trial**. In the context of measuring a drug response in non-infectious uveitis, a clinical trial is the most appropriate method for evaluating the efficacy of a specific medication on treating the condition. Clinical trials involve testing a new drug or treatment strategy in a controlled, systematic manner, usually comparing it to a placebo or an existing standard treatment.

**Why Each Wrong Option is Incorrect**:

A. **Option A (Case Series):** A case series involves describing a series of cases with a common feature or outcome. It does not provide a definitive answer on drug response but rather describes a series of cases.

B. **Option B (Case Report):** A case report is a brief description of an unusual case, often with a single case. It is not suitable for evaluating drug response in uveitis.

D. **Option D (Retrospective Study):** A retrospective study evaluates data from past medical records, usually to identify trends or associations, but not to assess drug response.

**Clinical Pearl**: Understanding the difference between these study designs is essential for medical professionals. While these study designs have their place in medical research, a clinical trial is specifically designed to assess the effectiveness of a drug or treatment strategy, making it the appropriate choice for measuring drug response in non-infectious uveitis.