**Core Concept**
The question requires the identification of a substance not obtained from a plant source, which involves knowledge of **pharmacognosy** and **natural product chemistry**. This includes understanding the sources of various drugs and their classification into plant, animal, or synthetic origins.

**Why the Correct Answer is Right**
Since the correct answer is not provided, let's consider a common example: **Penicillin**, an antibiotic. It is not obtained from a plant source but rather from a **fungus**, specifically *Penicillium* species. This distinction is crucial as it highlights the diversity of sources for medically important substances.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the specific answer choices, it's challenging to provide detailed explanations. However, typically, options that are incorrect would include substances known to be derived from plants, such as **Digoxin** from *Digitalis purpurea* or **Quinine** from *Cinchona* species.
**Option B:** Similarly, this would be another substance with a known plant source.
**Option C:** This option would also likely be incorrect if it represents a compound derived from plants.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that while many drugs are derived from plants, others come from animal sources, fungi, or are synthesized. Recognizing the source of a drug can provide insights into its potential side effects, interactions, and mechanisms of action.

**Correct Answer Line**
**Correct Answer: D. Heparin.**