Final answer:
The least useful method for treating discolored eyelids among the options is a hypodermic injection of a bleaching agent due to the delicate nature of the eyelid skin and potential for harm.
Step-by-step explanation:
The question is asking which of the listed methods would not be effective for treating discolored eyelids. Among the options given, hypodermic injection of bleaching agent is not a practical or safe approach for treating discoloration of eyelids. This is because the skin of the eyelids is very thin and sensitive, making an injection risky and likely to cause damage or complications. On the other hand, external pressure during embalming is a method used in funeral services to set features and may reduce discoloration temporarily by displacing blood, but it's not a treatment per se. The application of a surface compress with bleaching agent might be useful for treating hyperpigmentation if done carefully under professional guidance. Lastly, the use of application of opaque cosmetic can cover up discoloration but does not treat the underlying condition.