Which of the following is a common cause of pneumonia?

Pneumonia is primarily an infection that inflames the air sacs in one or both lungs, and it is commonly caused by various pathogens. Among these, viruses are a significant and prevalent cause of pneumonia, particularly in children and older adults. Viral pneumonia can be caused by pathogens such as the influenza virus, respiratory syncytial virus (RSV), and coronaviruses. The mechanism involves the virus infecting the respiratory epithelium, leading to inflammation and the accumulation of fluid in the air sacs.

In contrast, the other options presented do not directly cause pneumonia. Heart conditions can lead to complications that affect lung function, but they are not causes of pneumonia. Kidney stones are related to the urinary system and do not have a direct link to respiratory infections. Skin infections, while serious, typically do not lead to pneumonia and are not associated with the pathogens that primarily cause lung infections. Therefore, viruses are the most relevant answer in the context of common pneumonia causes.