**Core Concept**
The underlying principle being tested is the diagnosis of respiratory complications in pregnancy, specifically differentiating between asthma exacerbation and other potential causes of shortness of breath. **Pulmonary embolism (PE)** and **asthma** are two key considerations in this scenario, given the patient's history and presentation.

**Why the Correct Answer is Right**
Given the patient's symptoms of shortness of breath, tachypnea, and the absence of wheezing or other signs of asthma exacerbation, along with the chest x-ray findings (not described but implied to be relevant), the most likely diagnosis would be **pulmonary embolism (PE)**. The patient's **hypoxemia (low PO2)**, **tachypnea**, and **tachycardia** are consistent with PE, especially in the context of pregnancy, which increases the risk of venous thromboembolism.

**Why Each Wrong Option is Incorrect**
**Option A:** Would be incorrect if it suggested an asthma exacerbation, as the lung exam is clear, and the PEFR is relatively preserved.
**Option B:** Might be incorrect if it proposed a cardiac issue, as the heart exam does not indicate significant dysfunction or signs suggestive of heart failure.
**Option C:** Could be incorrect if it implied another diagnosis not supported by the clinical presentation or laboratory findings.
**Option D:** Would be incorrect if it suggested a diagnosis that does not align with the patient's symptoms and test results.

**Clinical Pearl / High-Yield Fact**
Pregnancy increases the risk of **venous thromboembolism**, including **pulmonary embolism**, due to hypercoagulability, venous stasis, and vascular wall injury. Thus, any pregnant patient presenting with acute onset of shortness of breath and tachypnea should prompt consideration of PE.

**Correct Answer:** D. Pulmonary Embolism