The size of the crystal lying next to the red blood cell is roughly 17.5 um (micrometers).
Red blood cells are the most numerous cells in the human body. They are often known as RBCs or are commonly called erythrocytes. They get their name from their bright red color, which they in turn get from a protein they contain called hemoglobin. This protein is responsible for carrying oxygen from the lungs to the various tissues of the body. Erythrocytes have a diameter that ranges from 6 to 8 micrometers.
In the question, it states that we observe an unknown crystal lying next to a red blood cell. Being unknown, we cannot directly know its size. However, the question offers context as to the size of the crystal by stating that it is 2.5 times the diameter of the red blood cell. Through this information, and with the diameter of the red blood cell ranging from 6 to 8 (7 on average), we can infer that the size of the crystal is roughly 17.5um.
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