You are seeing a patient in clinic who recently started treatment for active tuberculosis. The patient is currently being treated with rifampin, isoniazid, pyrazinamide, and ethambutol. The patient is not used to taking medicines and is very concerned about side effects. Specifically regarding the carbohydrate polymerization inhibiting medication, which of the following is a known side effect?

Paresthesias of the hands and feet

A 17-year-old girl is brought to the emergency department 6 hours after she attempted suicide by consuming 16 tablets of acetaminophen (500 mg per tablet). At present, she does not have any complaints or symptoms. The patient is afebrile and vital signs are within normal limits. Physical examination is unremarkable. Laboratory findings show a serum acetaminophen level that is predictive of ‘probable hepatic toxicity’ on the Rumack-Matthew nomogram. Treatment is started with a drug, which is a precursor of glutathione and is a specific antidote for acetaminophen poisoning. Which of the following is an additional beneficial mechanism of action of this drug in this patient?

Promotes microcirculatory blood flow

Promotes glucuronidation of unmetabolized acetaminophen

Promotes fecal excretion of unabsorbed acetaminophen

Prevents gastrointestinal absorption of acetaminophen

Promotes oxidation of N-acetyl-p-benzoquinoneimine (NAPQI)

A 46-year-old man comes to the physician for a follow-up examination. Two weeks ago, he underwent laparoscopic herniorrhaphy for an indirect inguinal hernia. During the procedure, a black liver was noted. He has a history of intermittent scleral icterus that resolved without treatment. Serum studies show: Aspartate aminotransferase 30 IU/L Alanine aminotransferase 35 IU/L Alkaline phosphatase 47 mg/dL Total bilirubin 1.7 mg/dL Direct bilirubin 1.1 mg/dL Which of the following is the most likely diagnosis?

Type II Crigler-Najjar syndrome

A 25-year-old woman presents with slightly yellow discoloration of her skin and eyes. She says she has had multiple episodes with similar symptoms before. She denies any recent history of nausea, fatigue, fever, or change in bowel/bladder habits. No significant past medical history. The patient is afebrile and vital signs are within normal limits. On physical examination, she is jaundiced, and her sclera is icteric. Laboratory findings are significant only for a mild unconjugated hyperbilirubinemia. The remainder of laboratory results is unremarkable. Which of the following is the most likely diagnosis in this patient?

Crigler-Najjar syndrome type II

A 35-year-old woman presents to the emergency department after ingesting approximately 50 tablets of acetaminophen 4 hours ago in a suicide attempt. Her acetaminophen level is 200 µg/mL. Which of the following best describes the mechanism of toxicity in this case?

Depletion of glutathione stores

Inhibition of cytochrome oxidase

An 18-month-old boy is brought in to the pediatrician by his mother for concerns that her child is becoming more and more yellow over the past two days. She additionally states that the boy has been getting over a stomach flu and has not been able to keep down any food. The boy does not have a history of neonatal jaundice. On exam, the patient appears slightly sluggish and jaundiced with icteric sclera. His temperature is 99.0°F (37.2°C), blood pressure is 88/56 mmHg, pulse is 110/min, and respirations are 22/min. His labs demonstrate an unconjugated hyperbilirubinemia of 16 mg/dL. It is determined that the best course of treatment for this patient is phenobarbital to increase liver enzyme synthesis. Which of the following best describes the molecular defect in this patient?

Missense mutation in the UGT1A1 gene

Silent mutation in the UGT1A1 gene

Nonsense mutation in the UGT1A1 gene

Mutation in the promoter region of the UGT1A1 gene

Phase II metabolism (conjugation reactions) for USMLE Step 2 CK: High-Yield Pharmacology Lessons

Phase II Reactions - The Cleanup Crew

Phase II reactions attach endogenous polar molecules to a drug or its Phase I metabolite. This conjugation step drastically increases water solubility, preparing the compound for elimination.

Primary Goal: ↑ Polarity & water solubility for excretion.
Major Pathways:
- Glucuronidation (most common, uses UDP-glucuronosyltransferase)
- Acetylation (e.g., Isoniazid, Hydralazine)
- Sulfation (e.g., Acetaminophen)
📌 Mnemonic: GAS (Glucuronidation, Acetylation, Sulfation).

⭐ High-Yield: Genetic polymorphism in N-acetyltransferase (NAT) leads to "slow acetylators" vs. "fast acetylators." Slow acetylators have ↑ risk of drug toxicity (e.g., drug-induced lupus from hydralazine, isoniazid-induced neuropathy).

Phase I and II Drug Metabolism Pathways

Conjugation Pathways - The Magnificent Six

Phase II reactions attach endogenous polar groups to drugs, making them more water-soluble for excretion. This process generally inactivates the drug.

Glucuronidation
- Enzyme: UGT (UDP-glucuronosyltransferase)
- Cofactor: UDP-glucuronic acid
- Most common pathway; deficient in neonates (e.g., Gray Baby Syndrome).
- Drugs: Morphine, Acetaminophen, Lorazepam.
Sulfation
- Enzyme: SULT (Sulfotransferase)
- Cofactor: PAPS
- Drugs: Acetaminophen, Methyldopa.
Acetylation
- Enzyme: NAT (N-acetyltransferase)
- Cofactor: Acetyl-CoA
- Drugs: Sulfonamides, Hydralazine, Isoniazid, Procainamide (📌 SHIP).
Glutathione Conjugation
- Enzyme: GST (Glutathione-S-transferase)
- Cofactor: Glutathione (GSH)
- Neutralizes reactive metabolites like NAPQI from acetaminophen.
Glycine Conjugation
- Metabolizes salicylates.
Methylation
- Enzyme: TPMT, COMT
- Cofactor: SAM
- Drugs: Azathioprine, 6-Mercaptopurine.

⭐ Slow vs. Fast Acetylators: Genetic polymorphism in NAT2 is a classic exam topic. Slow acetylators have ↑ risk of drug-induced lupus (e.g., Hydralazine, Procainamide) and peripheral neuropathy (Isoniazid).

Clinical Correlations - When Genes Stumble

UDP-Glucuronosyltransferase (UGT) Deficiency: Affects bilirubin & drug conjugation.
- Gilbert's Syndrome: Mildly ↓ UGT activity. Benign unconjugated hyperbilirubinemia, provoked by stress or illness.
- Crigler-Najjar, Type I: Absent UGT. Severe jaundice, kernicterus; fatal without liver transplant.
- Crigler-Najjar, Type II: Markedly ↓ UGT activity. Responds to phenobarbital (induces UGT).
- Gray Baby Syndrome: Neonatal UGT immaturity causes toxic chloramphenicol accumulation.
N-acetyltransferase (NAT) Polymorphism:
- Slow Acetylators: ↑ risk for drug toxicity.
  - Isoniazid: Peripheral neuropathy.
  - Hydralazine, Procainamide, Isoniazid: Drug-induced lupus. 📌 SHIP happens.
- Fast Acetylators: May need higher doses of drugs like isoniazid.

⭐ Slow acetylators are at increased risk for drug-induced lupus erythematosus (DILE) from hydralazine, procainamide, and isoniazid.

Bilirubin Metabolism and Glucuronidation Pathway

High-Yield Points - ⚡ Biggest Takeaways

Phase II reactions make drugs polar and water-soluble for renal excretion by adding endogenous substrates.

Key pathways: Glucuronidation (most common), Sulfation, Acetylation, and Glutathione conjugation.

Genetic polymorphism of N-acetyltransferase (NAT) creates slow/fast acetylators, affecting toxicity of drugs like isoniazid.

Neonatal UGT deficiency impairs drug/bilirubin conjugation, leading to jaundice and toxicity.

Glutathione (GSH) depletion is the mechanism of acetaminophen hepatotoxicity; treated with N-acetylcysteine (NAC).

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