**Core Concept**
Increased blood viscosity is a critical factor in the development of thrombosis and cardiovascular diseases. It is influenced by factors such as hematocrit, plasma protein concentration, and red blood cell deformability.

**Why the Correct Answer is Right**
Increased viscosity of blood can be caused by an elevated hematocrit, which occurs when there is a relative or absolute decrease in plasma volume. This can be seen in conditions such as dehydration, heart failure, or severe burns. As hematocrit increases, the concentration of red blood cells also increases, leading to increased blood viscosity.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not directly related to blood viscosity.
**Option B:** This option may actually decrease blood viscosity by increasing plasma volume.
**Option C:** While this option may affect blood pressure, it does not directly impact blood viscosity.

**Clinical Pearl / High-Yield Fact**
It's essential to note that increased blood viscosity can lead to decreased perfusion of organs, particularly the brain and kidneys, which can result in symptoms such as dizziness, confusion, and decreased urine output.

**Correct Answer: C. Increased plasma protein concentration. Increased plasma protein concentration can also contribute to increased blood viscosity by increasing the osmotic pressure and reducing plasma volume.**