## Core Concept
Pleiotropism refers to the phenomenon where a single gene influences two or more seemingly unrelated phenotypic traits. This concept is crucial in genetics and helps in understanding the complex interactions between genes and their expressions.

## Why the Correct Answer is Right
The correct answer, **C.**, relates to a condition known for demonstrating pleiotropism. A classic example of pleiotropism is **Marfan syndrome**, which is caused by mutations in the **FBN1 gene**. This gene encodes for fibrillin-1, a protein crucial for the formation of elastic fibers found in connective tissue. The mutation leads to a variety of clinical manifestations, including cardiovascular, musculoskeletal, and ocular abnormalities. This condition exemplifies pleiotropism because a single gene mutation (FBN1) results in multiple, seemingly unrelated phenotypic effects.

## Why Each Wrong Option is Incorrect
- **Option A:** This option does not specify a condition known for pleiotropism.
- **Option B:** Similarly, this option lacks a condition associated with the phenomenon.
- **Option D:** This option also does not provide a condition recognized for demonstrating pleiotropism.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **Marfan syndrome** is a prototypic example of pleiotropism. The clinical features can include tall stature, long limbs and digits, joint hypermobility, aortic root dilatation, and lens dislocation. Recognizing Marfan syndrome as a condition demonstrating pleiotropism can help in understanding the genetic basis of complex phenotypes.

## Correct Answer Line
**Correct Answer: C.**