**Core Concept**
Hemoconcentration refers to an increase in the concentration of red blood cells, white blood cells, and platelets in the blood due to a reduction in plasma volume. This phenomenon occurs in various clinical conditions where there is a significant loss of fluid from the circulatory system, leading to a relative increase in the concentration of blood cells.

**Why the Correct Answer is Right**
Hemoconcentration is typically seen in conditions characterized by severe dehydration, such as in shock states, severe burns, or gastrointestinal fluid loss. In these situations, there is a significant loss of plasma volume, which leads to a relative increase in the concentration of blood cells. This is because the volume of red blood cells remains relatively constant, while the plasma volume decreases. The kidneys play a crucial role in Hemoconcentration by concentrating the urine in response to decreased plasma volume, which further exacerbates the increase in blood cell concentration.

**Why Each Wrong Option is Incorrect**
**Option A:** Dehydration due to diarrhea is not a correct answer because, although it can lead to fluid loss, the loss of electrolytes and water is usually accompanied by a relatively small loss of plasma volume.
**Option B:** In chronic kidney disease, the kidneys lose their ability to concentrate urine, leading to a decrease in blood cell concentration, not an increase.
**Option C:** In severe anemia, the concentration of red blood cells in the blood actually decreases, not increases.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that hemoconcentration can lead to an increase in blood viscosity, which can further impair blood flow and increase the risk of thrombotic events.

**Correct Answer: D. Severe dehydration.**