Chronic Obstructive Pulmonary Disease (COPD) Exacerbation: This is the most likely diagnosis given the elevated pCO2, which indicates hypercapnia, a common complication of COPD exacerbations. The bilateral congestion on the chest X-ray (CXR) could be due to the increased mucus production and airway inflammation typical of COPD exacerbations.

Pneumonia: Bilateral lung congestion on CXR is a hallmark of pneumonia, which can also lead to hypercapnia, especially if the pneumonia is severe or if the patient has underlying lung disease.
Pulmonary Edema: This condition, often due to heart failure, can cause bilateral congestion on CXR and lead to hypercapnia due to impaired gas exchange.
Acute Respiratory Distress Syndrome (ARDS): Although more typically associated with hypoxemia, severe cases of ARDS can lead to hypercapnia, especially if there is significant lung injury and impaired ventilation.

Status Asthmaticus: A severe asthma attack can lead to hypercapnia due to severe airway obstruction and can present with bilateral lung congestion on CXR. Missing this diagnosis can be fatal.
Pulmonary Embolism: While not typically causing bilateral congestion, large pulmonary emboli can lead to acute right heart strain, which might result in elevated pCO2 due to decreased cardiac output and impaired ventilation-perfusion matching.

Cystic Fibrosis Exacerbation: Patients with cystic fibrosis can experience exacerbations that lead to increased mucus production, airway obstruction, and subsequently, hypercapnia. However, this would be less common in the absence of a known history of cystic fibrosis.
Neuromuscular Disease: Conditions like myasthenia gravis or Guillain-Barré syndrome can lead to respiratory failure, including hypercapnia, due to muscle weakness affecting the respiratory muscles. These conditions would typically present with additional neurological symptoms.