**Core Concept**
Hereditary spherocytosis is a genetic disorder characterized by the production of abnormal red blood cells (RBCs) that are sphere-shaped rather than biconcave disk-shaped. This abnormal shape leads to increased osmotic fragility, making the RBCs more prone to hemolysis (breakdown) in hypotonic solutions.

**Why the Correct Answer is Right**
In hereditary spherocytysis, mutations in genes encoding proteins of the RBC membrane, such as spectrin, ankyrin, band 3, or protein 4.2, disrupt the normal cytoskeletal structure and membrane stability. As a result, the sphere-shaped RBCs have a larger surface area-to-volume ratio, making them more susceptible to osmotic stress and subsequent lysis. This increased osmotic fragility is a hallmark of the disease, often leading to hemolytic anemia.

**Why Each Wrong Option is Incorrect**
**Option A:** Sickle cell anemia is characterized by a different pathophysiological mechanism, involving the abnormal polymerization of hemoglobin S under deoxygenated conditions, leading to sickling of RBCs. While RBCs in sickle cell anemia do have increased fragility, it is not due to osmotic fragility.

**Option B:** Thalassemia major is caused by mutations in the genes encoding alpha- or beta-globin chains of hemoglobin, leading to reduced or absent production of one or both chains. While thalassemia major does lead to RBC fragility, it is primarily due to the reduced hemoglobin content rather than increased osmotic fragility.

**Option D:** Chronic lead poisoning affects the production of heme and globin chains, leading to microcytic, hypochromic anemia. However, the increased osmotic fragility seen in lead poisoning is not as pronounced as in hereditary spherocytosis.

**Clinical Pearl / High-Yield Fact**
A key clinical feature of hereditary spherocytosis is the presence of jaundice, particularly in newborns, due to the increased hemolysis leading to elevated bilirubin levels.

**✓ Correct Answer: C. Hereditary spherocytosis**