Differential Diagnosis for a 17-Year-Old Woman with Shortness of Breath and History of Frequent Infections

Single Most Likely Diagnosis

• Pulmonary Agenesis or Hypoplasia: This condition, which involves the abnormal development of lung tissue, can lead to the findings described, including the deviation of mediastinal structures to the left and the elevation of the right hemidiaphragm due to the underdevelopment or absence of lung tissue on the left side. The history of frequent infections as a child and current shortness of breath are consistent with this diagnosis, as underdeveloped lung tissue can lead to reduced lung function and increased susceptibility to infections.

Other Likely Diagnoses

• Diaphragmatic Hernia: Although less likely given the patient's age and the fact that such conditions are typically diagnosed earlier in life, a diaphragmatic hernia could potentially cause similar findings if it led to herniation of abdominal contents into the thoracic cavity, displacing mediastinal structures and affecting lung development or function.
• Pneumonectomy or Lobectomy: Surgical removal of a lung or a lobe could result in similar radiographic findings, including the shift of mediastinal structures towards the side of the surgery. However, this would typically be known in the patient's history.

Do Not Miss Diagnoses

• Pulmonary Thromboembolism: Although the chest X-ray and blood tests do not directly suggest this, pulmonary thromboembolism can cause sudden onset of shortness of breath and can be life-threatening. It's essential to consider this diagnosis, especially if there are any risk factors for thromboembolism.
• Tension Pneumothorax: This is a medical emergency that could present with shortness of breath and could potentially cause deviation of mediastinal structures if large enough. Immediate recognition and treatment are crucial.

Rare Diagnoses

• Congenital Diaphragmatic Anomalies (e.g., Eventration of the Diaphragm): These are rare conditions where there is an abnormal formation of the diaphragm, which could potentially lead to some of the findings described, especially if there is associated lung hypoplasia or if the anomaly affects diaphragmatic function.
• Bronchial Atresia: This involves the abnormal development of a bronchus, which could potentially lead to underdevelopment of the affected lung segment or lobe, but it might not fully explain the extensive findings described unless it was part of a more complex congenital anomaly.

In terms of the specific question about abnormal development, the best explanation for these findings would involve the lung bud (pulmonary diverticulum), as issues with its development could lead to agenesis or hypoplasia of the lung, consistent with the clinical and radiographic presentation described.