**Core Concept**
A spontaneous pneumothorax occurs when air enters the pleural space, causing the lung to collapse. This can lead to an imbalance in intrathoracic pressure, affecting the mediastinal shift and cardiovascular system.

**Why the Correct Answer is Right**
In a spontaneous pneumothorax, the air enters the pleural space, which is a potential space between the visceral and parietal pleura. As the lung collapses, the intrathoracic pressure increases on the affected side, causing a shift of the mediastinum to the opposite side. This shift is often away from the affected lung, towards the healthy lung. The increased intrathoracic pressure also affects the venous return to the heart, leading to decreased cardiac output.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the immediate effects of a spontaneous pneumothorax on the cardiovascular system.
* **Option B:** This option is incorrect because the mediastinal shift is typically away from the affected lung, not towards it.
* **Option C:** This option is incorrect because the decrease in intrathoracic pressure is not a direct consequence of a spontaneous pneumothorax.

**Clinical Pearl / High-Yield Fact**
In cases of spontaneous pneumothorax, the patient may present with sudden onset of chest pain, shortness of breath, and decreased breath sounds on the affected side. A chest radiograph is essential for diagnosis, and immediate treatment with needle aspiration or chest tube insertion may be necessary.

**Correct Answer: B.** The mediastinum shifts away from the affected lung.