**Core Concept:** Positive Kehr's sign refers to a clinical examination finding that suggests pleural effusion, especially in the context of pneumonia. It is caused by the increased respiratory excursion in the affected hemithorax due to reduced lung compliance caused by pleural effusion.

**Why the Correct Answer is Right:** Positive Kehr's sign occurs when there is increased respiratory movement in the affected hemithorax during expiration, which is associated with pleural effusion. This sign is caused by the reduced lung compliance due to pleural effusion, leading to exaggerated movement of the affected hemithorax during exhalation.

**Why Each Wrong Option is Incorrect:**

A. **False:** Pleural friction rub is a distinct clinical finding characterized by a rough, grating sound heard over the affected hemithorax due to the interaction between the parietal and visceral pleura. Positive Kehr's sign is not the same as a pleural friction rub.

B. **False:** Pleural friction rub is another clinical finding associated with pleural effusion, not positive Kehr's sign. These two signs assess different aspects of pleural involvement and should not be confused.

C. **False:** This option is not specific to positive Kehr's sign but is a general description of increased respiratory movement during expiration. Positive Kehr's sign is a more specific sign associated with pleural effusion and reduced lung compliance.

D. **False:** This option is a general description of decreased respiratory movement during inspiration, not related to pleural effusion or positive Kehr's sign.

**Clinical Pearl:** Negative Pleuropulmonary examination findings can provide valuable information about pleural and pulmonary involvement. Recognizing these signs can help guide the diagnosis and management of patients with suspected pleural effusion or pneumonia.