A pleural effusion is accumulation of excessive fluid in the pleural space, the potential space that surrounds each lung. Under normal conditions, pleural fluid is secreted by the parietal pleural capillaries at a rate of 0.6 millilitre per kilogram weight per hour, and is cleared by lymphatic absorption leaving behind only 5–15 millilitres of fluid, which helps to maintain a functional vacuum between the parietal and visceral pleurae. Excess fluid within the pleural space can impair inspiration by upsetting the functional vacuum and hydrostatically increasing the resistance against lung expansion, resulting in a fully or partially collapsed lung. Various kinds of fluid can accumulate in the pleural space, such as serous fluid (hydrothorax), blood (hemothorax), pus (pyothorax, more commonly known as pleural empyema), chyle (chylothorax), or very rarely urine (urinothorax). When unspecified, the term "pleural effusion" normally refers to hydrothorax. A pleural effusion can also be compounded by a pneumothorax (accumulation of air in the pleural space), leading to a hydropneumothorax. Types
Various methods can be used to classify pleural fluid. By the origin of the fluid:

Serous fluid (hydrothorax)
Blood (haemothorax)
Chyle (chylothorax)
Pus (pyothorax or empyema)
Urine (urinothorax)By pathophysiology:

Transudative pleural effusion
Exudative pleural effusionBy the underlying cause (see next section). Causes
Transudative
The most common causes of transudative pleural effusion in the United States are heart failure and cirrhosis. Nephrotic syndrome, leading to the loss of large amounts of albumin in urine and resultant low albumin levels in the blood and reduced colloid osmotic pressure, is another less common cause of pleural effusion.
Differential diagnosis
It is important to differentiate infantile acne from other forms of acneiform eruptions. Acne venenata infantum is a form of acne characterized by comedone formation and induced by chemical irritants on the skin. This can include comedogenic products such as lotions, ointments, creams and oils. Upon discontinuation of these products, lesions will heal within 6–8 weeks.Other conditions that should be considered include periorificial dermatitis, keratosis pilaris, sebaceous hyperplasia and infections. Pyodermas and panniculitis should be considered in severe cases of inflammatory acne or in cases of acne conglobata while hyperandrogenism should be ruled out in cases of persistent infantile acne. Treatment
Infantile acne is a self-limiting condition that resolves by itself within 6–12 months of occurrence or occasionally by ages 4–5 and does not require treatment in most cases but topical therapies can be used, especially in more severe cases.The goals of treatment are to reduce sebum production, prevent formation of microcomedones, suppress the growth of bacteria and reduce inflammation. As there are no US FDA approved medications for treatment of infantile acne due to lack of high quality trials in patients under 9, recommendations for treatment is based on observations in adult and adolescent populations. Benzoyl peroxide
Benzoyl peroxide (BPO) is first line for mild cases of infantile acne due to its safety and effectiveness. BPO concentrates within cells of sebaceous follicles where it generates free radicals to oxidize proteins in bacteria such as P. acnes. This leads to bacterial death. It additionally works as a mild comedolytic and anti-inflammatory.