Vasovagal Syncope: This is a common cause of syncope in young adults, often triggered by stress, pain, or prolonged standing. The low WBC count could be incidental or related to a viral infection, which might also contribute to the syncope episode.

Dehydration or Volume Depletion: This can lead to syncope due to decreased blood volume and potentially cause a relative leukopenia. Dehydration can be due to various reasons such as inadequate fluid intake, excessive sweating, or gastrointestinal losses.
Anemia: Although not directly indicated by the information provided, anemia could cause syncope due to reduced oxygen delivery to tissues. It might also be associated with a low WBC count in certain conditions.
Infectious Diseases: Certain infections can cause both syncope (through various mechanisms like dehydration, toxin effects, or direct cardiac involvement) and leukopenia. Viral infections are a common cause of leukopenia.

Cardiac Arrhythmias: Conditions like long QT syndrome can cause syncope and might not have an obvious connection to the WBC count. These are critical to diagnose due to their potential for fatal outcomes.
Sepsis: Although the WBC count is low, sepsis can present with leukopenia, especially in immunocompromised patients or those with certain types of infections (e.g., viral infections, or specific bacterial infections like typhoid fever). Sepsis is a life-threatening condition that requires immediate recognition and treatment.
Hematologic Malignancies: Leukemia or lymphoma could present with leukopenia and potentially cause syncope through various mechanisms (anemia, thrombocytopenia, or direct cardiac involvement).

Autoimmune Disorders: Conditions like systemic lupus erythematosus (SLE) can cause leukopenia and potentially lead to syncope through various mechanisms, including vasculitis, cardiac involvement, or secondary antiphospholipid syndrome.
Nutritional Deficiencies: Severe deficiencies in nutrients like vitamin B12 or folate can lead to leukopenia and potentially contribute to syncope, especially if there's associated anemia.
Adrenal Insufficiency: This condition can cause syncope due to hypotension and might be associated with leukopenia. It's a rare but critical diagnosis to consider due to its potential for severe outcomes if not treated promptly.