**Core Concept**
Leukostasis is a life-threatening complication of leukemic blast crisis characterized by the obstruction of small blood vessels by an excessive number of white blood cells. This condition is often seen in patients with acute myeloid leukemia (AML) or acute lymphoblastic leukemia (ALL). The pathophysiology involves the sludging of leukemic cells in the microcirculation, leading to tissue hypoxia and organ dysfunction.

**Why the Correct Answer is Right**
The lung is the organ most commonly affected by leukostasis due to its high vascular surface area and the propensity for leukemic cells to accumulate in the pulmonary capillaries. This leads to respiratory failure, characterized by hypoxemia, and may progress to acute respiratory distress syndrome (ARDS). The leukemic cells can also cause direct damage to the lung tissue, exacerbating the condition. The mechanism involves the adhesion of leukemic cells to the endothelial lining of the pulmonary capillaries, leading to the formation of microthrombi and the activation of inflammatory pathways.

**Why Each Wrong Option is Incorrect**
**Option A:** The liver is not the primary organ affected in leukostasis, although it may be involved in the systemic consequences of the condition.
**Option B:** The heart is not directly affected by leukostasis, although it may be indirectly impacted by the systemic hypoxia and acidosis associated with the condition.
**Option C:** The kidneys may be affected by leukostasis, but they are not the primary organ involved in this condition.

**Clinical Pearl / High-Yield Fact**
Leukostasis is a medical emergency that requires prompt recognition and treatment, as it can rapidly progress to respiratory failure and death.

**Correct Answer: C. Lungs.