Final answer:
For a patient with congenital methemoglobinemia, Prilocaine is contraindicated because it can further increase methemoglobin levels, posing a risk to the patient.
Step-by-step explanation:
In the case of a 3-month-old patient with congenital methemoglobinemia undergoing a direct laryngoscopy and rigid bronchoscopy for the evaluation of stridor, the local anesthetic that is contraindicated is Prilocaine. Prilocaine can cause an increase in methemoglobin levels, which is problematic in patients with congenital methemoglobinemia. This condition leads to an abnormal level of methemoglobin, which reduces the oxygen-carrying capacity of the blood, making this medication potentially dangerous for the patient.