What distinguishes a bulla from other skin lesions?

A bulla is characterized specifically as a fluid-filled lesion that is greater than 1 cm in diameter. This distinguishing feature sets it apart from other types of skin lesions. For example, a flat lesion (which is what the first option describes) does not possess any elevation or filling, thereby excluding it from being classified as a bulla. Similarly, the second option describes a raised lesion that is less than 1 cm and lacks fluid, which also does not meet the criteria for a bulla. Lastly, the fourth option refers to a flat lesion that is greater than 1 cm, further distancing it from the definition of a bulla, which must indeed be raised and fluid-filled.

Understanding the distinctions in size and characteristics of these lesions is key in clinical assessments, as they inform diagnosis and potential treatment plans. The size and presence of fluid in a bulla make it unique among skin lesions and critical for proper identification in physical assessments.