**Core Concept:**
Pleural effusion is a condition characterized by the accumulation of fluid in the pleural space, which is the space between the two pleura (membranes that line the lungs and chest wall). This condition can be classified as transudate (due to normal physiological processes) or exudate (due to pathological processes). The pleural effusion can be detected clinically based on the amount and characteristics of the fluid.

**Why the Correct Answer is Right:**
The correct answer is **D.** 500-750 ml. Detecting pleural effusion clinically is based on the amount and characteristics of the fluid. The clinical cutoff for considering an effusion significant is when the amount of fluid is more than 500 ml. This amount is typically enough to cause symptoms such as dyspnea, chest pain, and decreased lung expansion.

**Why Each Wrong Option is Incorrect:**

A. 200 ml: This amount of fluid accumulation may not be significant enough to cause symptoms or cause changes in lung expansion and is usually within the normal range of pleural fluid accumulation due to physiological processes.

B. 1,000 ml: While this amount can cause symptoms and changes in lung expansion, it is not the clinical cutoff for considering an effusion significant.

C. All effusions: This option is incorrect because the clinical significance of pleural effusion depends on the amount and characteristics of the fluid, not just the presence of effusion.

**Clinical Pearl:**
A pleural effusion detected clinically requires further investigations, such as chest X-rays and possibly additional tests like chest CT or pleural fluid analysis, to determine the cause and appropriate management. Understanding when to suspect an effusion clinically is crucial for further investigation and management planning.

**Correct Answer:** . 500-750 ml