**Core Concept**
Plasma protein, particularly albumin, plays a crucial role in maintaining oncotic pressure, which is the pressure exerted by proteins in the blood vessels to retain fluid within the vascular space. When plasma protein levels decrease, the oncotic pressure decreases, leading to fluid shifting out of the blood vessels and into the interstitial space, resulting in edema.

**Why the Correct Answer is Right**
The correct answer is related to the concept of oncotic pressure and the role of albumin in maintaining it. Albumin is the most abundant plasma protein and is responsible for approximately 75% of the oncotic pressure in the blood vessels. When albumin levels decrease, the oncotic pressure decreases, leading to fluid shifting out of the blood vessels and into the interstitial space, resulting in edema. This is a critical concept in understanding the pathophysiology of edema.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the relationship between plasma protein levels and edema.
**Option B:** This option is incorrect because it does not accurately reflect the relationship between plasma protein levels and edema.
**Option C:** This option is incorrect because it does not provide a specific threshold for plasma protein levels that would lead to edema.

**Clinical Pearl / High-Yield Fact**
A decrease in plasma albumin levels below 3.5 g/dL is associated with a significant decrease in oncotic pressure, leading to edema. This is a critical threshold for clinicians to consider when evaluating patients with edema.

**Correct Answer: C. 3.5 g/dL**