**Core Concept**
The question is testing the student's knowledge of the sources of various medications or substances derived from marine organisms, specifically fish.

**Why the Correct Answer is Right**
The correct answer is likely "Vitamin C" because it is not derived from fish, unlike other options such as vitamin B12, omega-3 fatty acids, and cod liver oil, which are all obtained from fish or fish products. Vitamin C is primarily obtained from plant sources like fruits, vegetables, and leafy greens.

**Why Each Wrong Option is Incorrect**
**Option A:** Vitamin B12 is indeed obtained from fish, particularly from the liver, where it is stored in the form of methylcobalamin.
**Option B:** Omega-3 fatty acids, specifically EPA and DHA, are found in high concentrations in fatty fish like salmon and sardines.
**Option C:** Cod liver oil is a rich source of vitamin D and omega-3 fatty acids, both of which are essential nutrients obtained from fish.

**Clinical Pearl / High-Yield Fact**
It is essential for medical professionals to be aware of the importance of vitamin C in the prevention and treatment of scurvy, a disease caused by vitamin C deficiency, which can lead to bleeding gums, joint pain, and poor wound healing.

**Correct Answer: D. Vitamin C.**