The most common possible diagnoses for unexplained bruising are injuries the patient doesn't remember. It's not uncommon at all for an individual to suffer some minor bump during the day and forget about it ever happening. After that, they will be confused when they suddenly develop a bruise and they won't know why. There are also some very serious possibilities when a person has unexplained bruising, including various blood diseases and diabetes. Finally, old age is another possible cause of unexplained bruising, and some people get unexplained bruises because they workout too aggressively.
In the case of blood disease, many of them cause the blood to stop clotting properly, and this can result in bruising from very minor physical contact. Some of the disorders that might lead to this sort of unexplained bruising include hemophilia and leukemia. Many of these blood diseases are potentially deadly, and for that reason, it's generally considered very important for individuals to take unexplained bruises seriously.
Sometimes diabetics think that they're suffering with unexplained bruising, but in reality, the marks they find aren't actual bruises. Diabetics can develop patches of discolored skin on certain parts of their bodies, particularly in areas where the skin connects with other skin frequently. These patches are often darker, and the diabetic might think they are bruises, but these areas are actually related to an insulin-resistance-based condition.
In elderly people, the walls of their blood vessels are weaker than in younger people, and they generally have less fat to protect them from impact. This can make them very susceptible to bruising. Sometimes they can be bruised by very minor contact, and they won't have any memory of an injury. This kind of unexplained bruising is usually nothing to be concerned about, but it is generally considered wise for people to have it checked out as a precaution.
Body builders and other people who do a lot of weight lifting can injure themselves unknowingly, and that can result in bruises they can't explain. When people perform these kinds of exercises, they put a lot of strain on their muscles, and blood vessels can burst from the exertion. It's also generally true that any sort of vigorous exercise increases a person's chance for unexplained bruising. When people engage in physically taxing sporting activities, they tend to forget about minor physical impacts, and some of those impacts might be forceful enough to cause a bruise.