Which of the following is an intermediate-acting local anaesthetic which is an amino amide causing methemoglobinemia?
**Core Concept**
Prilocaine is a local anesthetic that belongs to the amino amide class. It is characterized by its intermediate duration of action, which is longer than that of lidocaine and shorter than that of bupivacaine. Prilocaine is metabolized by the liver, and its metabolites, including o-toluidine, can cause methemoglobinemia, a condition in which there is an abnormal amount of methemoglobin in the blood.
**Why the Correct Answer is Right**
Prilocaine's unique pharmacological profile makes it a popular choice for certain regional anesthesia procedures. Its intermediate duration of action allows for prolonged analgesia without the prolonged motor blockade associated with longer-acting local anesthetics. However, prilocaine's metabolism by the liver results in the production of o-toluidine, which is a potent oxidizing agent that can oxidize hemoglobin to methemoglobin. This can lead to symptoms such as cyanosis, shortness of breath, and even cardiac arrest in severe cases.
**Why Each Wrong Option is Incorrect**
**Option A:** Procaine is a short-acting local anesthetic that belongs to the amino ester class, not the amino amide class. It is not associated with methemoglobinemia.
**Option C:** Etidocaine is a long-acting local anesthetic that also belongs to the amino amide class. While it is associated with a slower onset of action, it is not known to cause methemoglobinemia.
**Option D:** Ropivacaine is a long-acting local anesthetic that belongs to the amino amide class. It is known for its reduced toxicity and is not associated with methemoglobinemia.
**Clinical Pearl / High-Yield Fact**
Prilocaine's potential to cause methemoglobinemia is a classic exam trap. To remember this, students can recall that prilocaine is the only local anesthetic that can cause methemoglobinemia due to its unique metabolism. This makes it a high-yield fact for exam purposes.
**✓ Correct Answer: B. Prilocaine**