**Core Concept**
The patient's presentation of dyspnea, recent stroke, and ECG changes of S1 Q3 T3 pattern, along with a V/Q scan showing major areas of reduced perfusion, suggests a diagnosis related to pulmonary embolism (PE) in the setting of a hypercoagulable state.

**Why the Correct Answer is Right**
The patient's recent stroke increases the risk of pulmonary embolism due to immobility and possible underlying hypercoagulability. The ECG changes of S1 Q3 T3 pattern are suggestive of right ventricular strain, which is consistent with a large pulmonary embolism. The V/Q scan showing major areas of reduced perfusion further supports this diagnosis, as it indicates areas of the lung with reduced blood flow. This is consistent with a pulmonary embolism causing a blockage in the pulmonary arteries.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not directly related to the patient's presentation of dyspnea, recent stroke, and ECG changes. Pneumonia is not supported by the V/Q scan findings.
**Option B:** Chronic obstructive pulmonary disease (COPD) may cause dyspnea, but it does not explain the recent stroke or ECG changes.
**Option C:** Cardiac arrhythmias can cause dyspnea, but they do not explain the V/Q scan findings or the recent stroke.

**Clinical Pearl / High-Yield Fact**
In patients with recent stroke, it is essential to consider pulmonary embolism as a potential complication, especially if they present with dyspnea and ECG changes suggestive of right ventricular strain.

**Correct Answer:** D.