**Core Concept**
This question assesses the understanding of the study design used in clinical research. A prospective study involves observing outcomes in subjects who are being exposed to a particular intervention, in this case, a new drug.

**Why the Correct Answer is Right**
This study is an example of a **prospective cohort study**. In this design, a group of patients (the cohort) is followed over time to assess the development of a specific outcome, in this case, cataract, while they are exposed to the new drug. The study aims to establish a temporal relationship between the exposure (drug administration) and the outcome (cataract development). The study is prospective because the data collection occurs before the outcome is observed.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a **case-control study** would involve selecting patients with cataract (cases) and comparing them to patients without cataract (controls) to assess the association between the new drug and cataract. This design is retrospective, and the data collection occurs after the outcome is observed.
**Option B:** This option is incorrect because a **cross-sectional study** would involve observing a defined population at a single point in time to assess the association between the new drug and cataract. This design does not involve following patients over time.
**Option C:** This option is incorrect because an **experimental study** would involve randomly assigning patients to receive either the new drug or a placebo, and then comparing the outcomes between the two groups. While this study is randomized, it is not an experimental study because the patients are not assigned to receive a placebo.

**Clinical Pearl / High-Yield Fact**
It is essential to understand the different study designs used in clinical research to critically evaluate the validity and generalizability of study findings. Knowing the strengths and limitations of each design can help clinicians make informed decisions when interpreting research evidence.

**Correct Answer:** C. Prospective cohort study.