What is the primary condition indicated by the presence of bright red blood under the conjunctiva?

The primary condition indicated by the presence of bright red blood under the conjunctiva is subconjunctival hemorrhage. This occurs when small blood vessels in the conjunctiva break, leading to bleeding beneath the transparent membrane that covers the white part of the eye. The blood appears as a sharply demarcated red patch on the sclera and is typically not associated with pain or vision changes. Subconjunctival hemorrhages can result from a variety of factors, including trauma, sudden increases in blood pressure (like coughing or straining), or can occur spontaneously without a clear cause.

In contrast, conjunctivitis usually presents with redness, discharge, and discomfort, but it does not specifically cause bright red patches of blood. Blepharitis involves inflammation of the eyelid margins, leading to irritation and crusting, but does not result in blood under the conjunctiva. Retinal detachment is a more serious condition characterized by symptoms such as sudden vision changes or the appearance of floaters and flashes, but it does not present with bright red blood under the conjunctiva either. Therefore, subconjunctival hemorrhage is the condition that aligns clearly with the described presentation.