Dehydration: This is the most likely cause of elevated red blood cell (RBC) count and hematocrit (HCT) in a 14-year-old female with a normal rest of the complete blood count (CBC). Dehydration leads to a decrease in plasma volume, which concentrates the blood and thus elevates the RBC count and HCT.

Diuretic Use or Abuse: The use of diuretics can lead to dehydration, which in turn can cause an elevation in RBC count and HCT.
High-Altitude Exposure: Living at or traveling to high altitudes can stimulate erythropoiesis, leading to an increase in RBC production and, consequently, elevated HCT.
Athletic Training or Endurance Sports: Intensive athletic training, especially in endurance sports, can lead to an increase in RBC mass and HCT due to increased erythropoietin production.

Polycythemia Vera: A myeloproliferative disorder that leads to an overproduction of RBCs, white blood cells, and platelets. Although less common in adolescents, it's crucial to rule out due to its potential for serious complications.
Chronic Lung Disease: Conditions like chronic obstructive pulmonary disease (COPD) or cystic fibrosis can lead to hypoxia, stimulating erythropoietin production and resulting in elevated RBC count and HCT.
Renal Tumors or Cysts: Certain renal abnormalities can produce excess erythropoietin, leading to secondary polycythemia.

Testosterone or Erythropoietin Abuse: Although more common in adult athletes, the abuse of testosterone or erythropoietin to enhance athletic performance can also occur in adolescents and would lead to elevated RBC counts and HCT.
Congenital Heart Disease with Right-to-Left Shunt: Conditions like tetralogy of Fallot can lead to chronic hypoxia, stimulating erythropoiesis and resulting in elevated HCT.
Familial Erythrocytosis: Rare genetic conditions that affect the regulation of erythropoietin production or its receptor, leading to increased RBC production.