Infectious Mononucleosis (Mono) due to Epstein-Barr Virus (EBV): The patient has a positive mononucleosis heterophile antibody test and an elevated EBV viral capsid antigen (VCA) IgG level, indicating a current or recent EBV infection. The symptoms of significant fatigue, along with the laboratory findings, strongly support this diagnosis.

Sleep Apnea Complications: Although the patient is on CPAP for Obstructive Sleep Apnea (OSA), inadequate control or non-compliance could contribute to fatigue. The history of Tetralogy of Fallot might also play a role in sleep disturbances or fatigue due to potential cardiac complications.
Anemia or Iron Deficiency: Despite normal iron and ferritin levels, the patient's high hemoglobin and hematocrit could be compensatory mechanisms. However, given the normal iron studies, this is less likely.
Hypothyroidism: Although the TSH level is normal, some patients with fatigue might have thyroid dysfunction not captured by a single TSH measurement. Further evaluation might be needed if symptoms persist.

Lymphoma: Given the patient's positive EBV IgG and the potential for EBV to be associated with certain lymphomas, it's crucial to monitor for any signs of lymphoma, especially if fatigue persists or worsens.
Chronic Fatigue Syndrome: This diagnosis should be considered if the patient's fatigue does not improve with treatment of the underlying cause (e.g., EBV infection) and other causes are ruled out.
Sleep Disorder: Other sleep disorders (e.g., insomnia, restless legs syndrome) could contribute to fatigue and should be evaluated, especially given the patient's history of OSA.

Hemochromatosis: Although the iron studies are normal, rare cases of hemochromatosis could present with fatigue among other symptoms. This would be more likely if there were a family history or other suggestive findings.
Mitochondrial Myopathies: These are rare disorders that affect the mitochondria and can cause fatigue among a wide range of other symptoms. They would be considered if other diagnoses are ruled out and there are specific clinical findings suggestive of a mitochondrial disorder.
Adrenal Insufficiency: This condition can cause fatigue and would be considered if other signs such as hypotension, hyponatremia, or hyperkalemia are present, although the patient's electrolytes are currently normal.