## Core Concept
The question revolves around the physiological changes and laboratory findings in a patient with congestive heart failure (CHF) treated with furosemide, a loop diuretic. The key concept here involves understanding the effects of diuresis on blood volume, electrolyte balance, and hematocrit levels.

## Why the Correct Answer is Right
The patient's increase in RBC count from 4 million/mm^3 to 7 million/mm^3 after diuresis with furosemide can be attributed to **hemoconcentration**. Furosemide induces a significant loss of water and electrolytes, leading to a decrease in plasma volume. Since the RBC count increased substantially, it indicates that the plasma volume decreased more significantly than the total blood volume, concentrating the red blood cells. This phenomenon is a classic example of hemoconcentration due to diuretic-induced plasma volume contraction.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is not provided, but typically, incorrect options might suggest other mechanisms such as increased erythropoiesis, which is not directly related to acute diuresis.
- **Option B:** Similarly, not provided, but could potentially suggest another pathological process not directly linked to the acute effects of furosemide.
- **Option C:** Without the specific details of option C, it's hard to address directly, but any option not suggesting hemoconcentration would be incorrect given the context.
- **Option D:** Assuming option D does not correctly identify hemoconcentration as the cause, it would be incorrect.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **diuretics can cause hemoconcentration**, leading to an increase in hematocrit and RBC count due to the reduction in plasma volume. This effect is particularly relevant in patients with CHF who are on diuretic therapy, as it can sometimes mask the severity of anemia or lead to spurious elevations in laboratory values that are volume-dependent.

## Correct Answer: D. Hemoconcentration.