**Question:** Which one of the following parameters is consistent with pleural transudate?

A. Protein concentration (PC) > 35 g/L
B. Lactate dehydrogenase (LDH) level > 20% of serum LDH
C. PC ≤35 g/L
D. PC ≤20 g/L

**Correct Answer:** D. PC ≤20 g/L

**Core Concept:** Pleural effusion is a collection of fluid in the pleural cavity surrounding the lungs. Pleural transudate is a type of pleural effusion characterized by the fluid being produced by the adjacent tissues and having a low protein concentration (PC) and low lactate dehydrogenase (LDH) level compared to the corresponding serum values.

**Why the Correct Answer is Right:** A pleural transudate is defined by a PC ≤20 g/L and a LDH level ≤20% of the serum LDH. This is consistent with a fluid that is produced by the adjacent tissues, typically due to hypoalbuminemia or reduced cardiac output.

**Why Each Wrong Option is Incorrect:**

A. Protein concentration (PC) > 35 g/L: This option introduces an arbitrary value for PC, which is not the defining criteria for pleural transudate. The correct criteria are PC ≤20 g/L and LDH ≤20% of serum LDH.

B. Lactate dehydrogenase (LDH) level > 20% of serum LDH: This option is incorrect because a pleural transudate is defined by a LDH level ≤20% of the serum LDH.

C. Protein concentration (PC) ≤35 g/L: This option is still too broad and does not accurately define the criteria for pleural transudate. The correct criteria are PC ≤20 g/L and LDH ≤20% of serum LDH.

D. Protein concentration (PC) ≤20 g/L: This option is the correct answer, as a PC of ≤20 g/L is consistent with a fluid that is produced by the adjacent tissues, typically due to hypoalbuminemia or reduced cardiac output.

**Clinical Pearls:**

1. A pleural transudate is a type of pleural effusion, which is a collection of fluid in the pleural cavity surrounding the lungs. Transudates are further classified into exudates (PC >35 g/L) and transudates (PC ≤35 g/L).
2. Hypoalbuminemia (low serum albumin) and reduced cardiac output are the two primary causes of pleural transudate formation.
3. Hypoalbuminemia leads to increased hydrostatic pressure in the pleural space, causing fluid filtration from the interstitial space into the pleural cavity.
4. Reduced cardiac output results in decreased filtration of proteins into the pleural space, leading to a low protein concentration in the