Postural Orthostatic Tachycardia Syndrome (POTS): This condition is characterized by a rapid increase in heart rate and other symptoms that occur upon standing, which can include dizziness, lightheadedness, and fainting. The patient's history of dizziness that worsens with activity and improves with rest, along with the lack of improvement after ablation for Wolff-Parkinson-White syndrome, makes POTS a plausible diagnosis. The focusing issues could be related to decreased cerebral blood flow during episodes of orthostatic intolerance.

Vestibular Migraine: Although the patient does not experience room spinning, vestibular migraines can present with a variety of symptoms including dizziness without vertigo, and can be triggered by activity. The history of focusing issues could be related to migraine-related visual disturbances.
Anxiety-Related Dizziness: Chronic anxiety can cause or exacerbate dizziness, and the patient's long-standing symptoms without a clear cause could suggest a psychological component. The worsening of symptoms with activity like playing basketball could be due to increased anxiety levels during such activities.
Deconditioning or Autonomic Dysfunction: Given the patient's history of Wolff-Parkinson-White syndrome and ablation, there could be underlying autonomic nervous system dysfunction contributing to his symptoms. Deconditioning from avoiding activities due to dizziness could also play a role.

Cardiac Causes (e.g., Cardiomyopathy, Arrhythmias): Although the patient had an ablation for Wolff-Parkinson-White syndrome, other cardiac conditions could be causing his symptoms. It's crucial to rule out any cardiac issues that could lead to dizziness, especially those that could be life-threatening.
Multiple Sclerosis or Other Demyelinating Diseases: These conditions can cause a wide range of neurological symptoms, including dizziness and visual disturbances. Early diagnosis is critical for management and prognosis.
Neurodegenerative Disorders (e.g., Friedreich's Ataxia): Certain neurodegenerative diseases can present with chronic dizziness and ataxia. Although less common, these diagnoses are important to consider due to their significant impact on the patient's quality of life and prognosis.

Chiari Malformation: A structural defect in the cerebellum that can cause dizziness, among other symptoms, due to compression of the brainstem and cerebellum. This condition is rare but could explain some of the patient's symptoms, especially if there are associated headaches or neck pain.
Mitochondrial Disorders: These are a group of disorders caused by dysfunctional mitochondria, which can affect any system in the body, including the nervous system, and might present with a variety of symptoms including dizziness, especially with exertion.