**Question:** Decreased osmotic fragility is seen in -

A. Hemolytic anemia
B. Polycythemia
C. Thalassemia
D. Sickle cell anemia

**Core Concept:** Osmotic fragility is a diagnostic test for hemolytic anemias, which measures the susceptibility of red blood cells (RBCs) to hemolysis when exposed to hypotonic solutions (low osmolarity). Hemolytic anemias are characterized by the premature breakdown of RBCs, leading to a decrease in RBC count and hemoglobin levels.

**Why the Correct Answer is Right:** Decreased osmotic fragility is observed in situations where RBCs are resistant to lysis, typically due to structural abnormalities or defects. In the case of polycythemia, increased RBC mass and volume result in elevated hematocrit levels, making RBCs more resistant to osmotic lysis, leading to decreased osmotic fragility.

**Why Each Wrong Option is Incorrect:**

A. Hemolytic anemia: Decreased osmotic fragility is a hallmark of hemolytic anemias, not a feature of them. In hemolytic anemias, RBCs are fragile and break down prematurely, leading to increased osmotic fragility.

B. Polycythemia: As mentioned, decreased osmotic fragility is observed in polycythemia, not hemolytic anemia.

C. Thalassemia: Thalassemia is characterized by abnormal production of hemoglobin, not altered RBC osmotic fragility.

D. Sickle cell anemia: In sickle cell anemia, RBCs exhibit deformability issues due to abnormal hemoglobin (HbS) and become rigid, leading to increased osmotic fragility, not decreased osmotic fragility.

**Clinical Pearl:** This concept helps in identifying the type of anemia based on osmotic fragility test results. For example, increased osmotic fragility indicates hemolytic anemia, while decreased osmotic fragility points towards polycythemia or other disorders causing elevated RBC volume.