**Core Concept**
Primary pulmonary hypertension (PPH) is a rare and idiopathic disorder characterized by elevated pulmonary artery pressure without any identifiable cause or underlying disease. It is a clinical diagnosis, and the absence of secondary causes is a hallmark of PPH.

**Why the Correct Answer is Right**
The correct answer is related to the fact that primary pulmonary hypertension is a diagnosis of exclusion, and various secondary causes of pulmonary hypertension must be ruled out before making the diagnosis. Secondary causes of pulmonary hypertension include left heart disease, lung diseases, sleep apnea, and thromboembolic disease. The absence of these secondary causes is a key feature of primary pulmonary hypertension.

**Why Each Wrong Option is Incorrect**
**Option A:** Pulmonary embolism is a common cause of secondary pulmonary hypertension, and it is often seen in patients with a history of deep vein thrombosis or thromboembolic disease.
**Option B:** Left heart disease, such as left ventricular dysfunction or mitral stenosis, can lead to secondary pulmonary hypertension, making this option incorrect.
**Option C:** Chronic obstructive pulmonary disease (COPD) is a common cause of secondary pulmonary hypertension, and it is often seen in patients with a history of smoking or occupational exposure to lung irritants.

**Clinical Pearl / High-Yield Fact**
Remember the mnemonic "PHILS" to help you remember the common secondary causes of pulmonary hypertension: Pulmonary Embolism, Left Heart disease, Idiopathic Pulmonary Fibrosis, and Sleep Apnea.

**Correct Answer: D.**