Create the text "Medical Alert". Attach conditional formatting to it which results in boolean true. When not true, the text can disappear. Make your text the color and size you want it when MedicalAttention is yes. Then under conditional formatting, do this:
ADD > (and under Condition), Formula is: MedicalAttention ≠ "Yes"
... where MedicalAttention is your field. Now down in format, select text color and make the text the exact same color as your layout background. It will disappear when MedicalAttention isn't Yes. :^)