**Core Concept**
Pulmonary hypertension is a condition characterized by elevated blood pressure in the pulmonary arteries, leading to right heart strain. In pregnancy, it can be caused by various factors, including increased blood volume and cardiac output, which can lead to increased pressure on the pulmonary arteries.

**Why the Correct Answer is Right**
The most likely cause of this patient's clinical presentation is **pulmonary hypertension**. This is due to the increased demand on the cardiovascular system during pregnancy, which can lead to an increase in pulmonary artery pressure. The lack of symptoms such as chest pain, cough, or fever, and the absence of known cardiac disease, make this a more likely diagnosis. Additionally, the patient's normal pulse, blood pressure, and oxygen saturation levels on examination do not suggest cardiac failure or other acute cardiac conditions.

**Why Each Wrong Option is Incorrect**
**Option A:** **Peripartum cardiomyopathy** is a condition that typically presents in the last month of pregnancy or within 5 months postpartum, with symptoms of heart failure. This patient's presentation in the 2nd trimester makes this diagnosis less likely.
**Option B:** **Pulmonary embolism** typically presents with sudden onset dyspnea, chest pain, and tachycardia, which are not present in this patient. The normal chest examination also makes this diagnosis less likely.
**Option C:** **Asthma** is unlikely given the absence of wheezing or cough, and the normal chest examination.
**Option D:** **Hypothyroidism** is not directly related to the patient's symptoms of dyspnea on exertion.

**Clinical Pearl / High-Yield Fact**
Pulmonary hypertension in pregnancy can be a life-threatening condition, and prompt recognition and management are essential to prevent complications. The patient's symptoms may progress rapidly, and it is essential to rule out other causes of dyspnea, such as cardiac failure or pulmonary embolism.

**Correct Answer:** C. Pulmonary hypertension.