Viral Myopericarditis or Viral Pneumonia: Given the patient's age, symptoms of tachycardia, tachypnea, hypoxia, chest pain, body aches, and the absence of elevated troponin or BNP, a viral etiology is plausible. The normal troponin and BNP levels decrease the likelihood of acute coronary syndrome or heart failure, pointing more towards an inflammatory or infectious cause.

Pulmonary Embolism (PE): Although the CXR is normal, PE can present with tachycardia, tachypnea, hypoxia, and chest pain. The absence of specific findings on CXR does not rule out PE, and the clinical presentation warrants consideration.
Acute Bronchitis or Pneumonia: These conditions could explain the respiratory symptoms and the elevated WBC count, indicating an infectious or inflammatory process.
Influenza or Other Viral Respiratory Infections: Given the body aches, fever (implied by the elevated WBC), and respiratory symptoms, a viral respiratory infection is a strong consideration.

Pneumothorax: Although the CXR is reported as normal, a pneumothorax can sometimes be missed, especially if it's small. The symptoms of chest pain and shortness of breath could be indicative of a pneumothorax, which is a medical emergency.
Cardiac Tamponade: Despite normal troponin and BNP, cardiac tamponade can present with tachycardia, hypotension (not mentioned but could be present), and chest pain. It's a life-threatening condition that requires immediate diagnosis and treatment.
Sepsis: The elevated WBC count and the presence of hypoxia and tachycardia could indicate sepsis, especially if there's a source of infection. Sepsis is a medical emergency that requires prompt recognition and treatment.

Lymphoma or Leukemia: Although rare in this age group and presentation, lymphoma or leukemia could explain the systemic symptoms such as body aches, nausea, and an elevated WBC count.
Sarcoidosis: This condition can present with a wide range of symptoms including chest pain, shortness of breath, and systemic symptoms like body aches. It's less common but should be considered in the differential diagnosis of a young patient with multi-system symptoms.
Vasculitis (e.g., Giant Cell Arteritis, Takayasu Arteritis): These conditions are rare and can present with systemic symptoms and chest pain. They are more commonly seen in older adults but can occur in younger individuals.