**Core Concept**
Reticulocytosis is the increase in the number of reticulocytes, immature red blood cells, in the peripheral blood. This phenomenon is often seen in conditions where there is an increased demand for erythropoiesis, the process of red blood cell production, or when there is an attempt to compensate for anemia. Reticulocytosis is typically observed in cases of hemolytic anemia, where red blood cells are being destroyed faster than they can be produced.

**Why the Correct Answer is Right**
In cases of hemolytic anemia, such as autoimmune hemolytic anemia (AIHA) or hemolytic disease of the newborn (HDN), the body attempts to compensate for the loss of red blood cells by increasing the production of new ones. This leads to an increase in reticulocytes in the peripheral blood, as the bone marrow responds to the increased demand for erythropoiesis. The reticulocytosis is a sign of the bone marrow's attempt to compensate for the anemia.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because anemia without reticulocytosis is typically seen in cases of iron deficiency anemia, where the body lacks sufficient iron to produce hemoglobin.
* **Option B:** This option is incorrect because anemia of chronic disease, also known as anemia of inflammation, is characterized by a decrease in erythropoietin production and an increase in hepcidin, leading to a decrease in iron availability for erythropoiesis.
* **Option C:** This option is incorrect because thalassemia major is a genetic disorder that affects hemoglobin production, leading to severe anemia, but it is not typically associated with reticulocytosis.

**Clinical Pearl / High-Yield Fact**
Reticulocytosis is a sign of the body's attempt to compensate for anemia, and it is often seen in cases of hemolytic anemia. A key clinical pearl to remember is that reticulocytosis is a sign of active erythropoiesis, and it can help differentiate between different types of anemia.

**Correct Answer: C. Thalassemia major is a genetic disorder that affects hemoglobin production, leading to severe anemia, but it is not typically associated with reticulocytosis.**