Question

if the pronoun in bold is vague, select all of the nouns or noun phrases it could refer to. if it is not vague, select its one clear antecedent.
medical technology may one day allow a doctor to identify genetic markers in a patients blood and use them to diagnose cancer before any symptoms are present.

Explanation:

The pronoun "them" clearly refers to the "genetic markers" that a doctor can identify in a patient's blood. There is no vagueness as the context makes it evident that the doctor will use the genetic markers for cancer - diagnosis.

Answer:

genetic markers