**Core Concept:** Acute exacerbation of chronic obstructive pulmonary disease (COPD), also known as acute respiratory distress due to COPD, is a life-threatening emergency in patients with pre-existing obstructive lung disease. The differential diagnosis includes pulmonary embolism, pneumonia, and asthma exacerbation. The diagnosis is mostly clinical, and certain investigations may be required to confirm the diagnosis and rule out other potential causes.

**Why the Correct Answer is Right:** In this case, the patient presents with severe acute breathlessness, but chest X-ray shows no focal consolidation, ruling out pneumonia. The patient is afebrile, suggesting that the acute exacerbation is not due to pyogenic infection. Elevated D-dimers suggest a hypercoagulable state, which is more consistent with a pulmonary embolism. However, a clinical diagnosis of acute exacerbation of COPD remains the most likely cause of the patient's symptoms.

**Why Each Wrong Option is Incorrect:**

A. Chest X-ray (option A) is initially helpful in ruling out pneumonia but not definitive for the diagnosis of acute exacerbation of COPD.

B. Blood culture (option B) is essential in ruling out pneumonia, but it is not helpful in diagnosing acute exacerbation of COPD or ruling out pulmonary embolism.

C. Chest CT (option C) is more sensitive and specific than chest X-ray for pneumonia but still not definitive for acute exacerbation of COPD.

D. D-dimer (option D) is helpful in ruling out pulmonary embolism, which is a possible differential diagnosis. However, elevated D-dimers do not confirm the diagnosis of acute exacerbation of COPD.

**Clinical Pearl:** In patients with acute exacerbation of COPD, a combination of clinical presentation, elevated D-dimers, and normal chest X-ray are suggestive of acute exacerbation of COPD rather than pulmonary embolism. However, a definitive diagnosis requires ruling out other potential causes, such as pneumonia and pulmonary embolism. A high index of suspicion and clinical acumen are essential in making a correct diagnosis in such cases, as the presented options are not definitive diagnostic tools for acute exacerbation of COPD.