**Core Concept**
Physiological anemia of infancy is a normal condition characterized by a decrease in hemoglobin levels in healthy full-term newborns. This occurs due to the adaptation of the infant's body to the transition from the high oxygen environment of the womb to the lower oxygen levels of the outside world. The reduction in erythropoietin production and the increased breakdown of red blood cells contribute to this decrease.

**Why the Correct Answer is Right**
Physiological anemia is a common finding in healthy full-term infants between 2-6 weeks of age, with a typical hemoglobin level of 10-12 g/dL. This decrease in hemoglobin is a result of the adaptation to the postnatal environment, which involves a decrease in erythropoietin production. Erythropoietin is a hormone produced by the kidneys that stimulates the production of red blood cells in the bone marrow. In the womb, erythropoietin levels are high to accommodate the high oxygen demands of the fetus. After birth, erythropoietin levels decrease, leading to a reduction in red blood cell production and a decrease in hemoglobin levels. This decrease in hemoglobin is a normal and expected finding in healthy infants.

**Why Each Wrong Option is Incorrect**
**Option B:** Pathological anemia refers to a decrease in hemoglobin levels that is caused by a underlying disease or condition. In this case, the infant's hemoglobin level is within the normal range for physiological anemia, so pathological anemia is not the correct diagnosis.
**Option C:** Thalassemia is a genetic disorder that affects the production of hemoglobin and can cause a decrease in hemoglobin levels. However, thalassemia typically presents with more severe anemia and other characteristic features, such as microcytosis and an elevated red blood cell count.
**Option D:** Iron deficiency anemia occurs when the body does not have enough iron to produce hemoglobin. However, iron deficiency anemia typically presents with a more significant decrease in hemoglobin levels and other characteristic features, such as microcytosis and an elevated total iron-binding capacity.

**Clinical Pearl / High-Yield Fact**
Physiological anemia of infancy is a normal and expected finding in healthy full-term infants between 2-6 weeks of age. It is essential to distinguish physiological anemia from pathological anemia, as the latter may require further investigation and treatment.

**✓ Correct Answer: A. Physiological anemia**