Master Biochemistry
for PMDC NLE Step 1

What the PMDC NLE Step 1 Tests in Biochemistry

The PMDC NLE Step 1 Biochemistry exam tests your ability to integrate molecular mechanisms with clinical presentations. You must recognise inborn errors of metabolism from neonatal crises (e.g., hypoglycaemia, metabolic acidosis, hyperammonaemia) and link enzyme deficiencies to specific laboratory findings (e.g., elevated succinylacetone in tyrosinaemia type I). Questions frequently require selecting the correct first-line treatment (e.g., N-carbamylglutamate for NAGS deficiency) or interpreting urine organic acids (e.g., methylmalonic acid in MMA). You must also apply vitamin cofactor knowledge (e.g., thiamine for maple syrup urine disease) and calculate anion gaps. Drug mechanisms (e.g., methotrexate inhibits dihydrofolate reductase) and genetic principles (e.g., autosomal recessive inheritance of PKU) are tested via patient scenarios. Emphasis is on immediate management decisions, not rote pathways.

High-Yield Concepts

• Maple Syrup Urine Disease (MSUD): Autosomal recessive deficiency of branched-chain alpha-ketoacid dehydrogenase. Presents in neonates with poor feeding, lethargy, and maple syrup odour. Key labs: elevated plasma leucine, isoleucine, valine; urine positive for branched-chain ketoacids. First-line: haemodialysis for severe encephalopathy, then thiamine (50-100 mg/day) in thiamine-responsive forms, and a branched-chain amino acid-restricted diet. Do not delay treatment pending confirmatory tests.
• Urea Cycle Disorders (UCDs): Most common: ornithine transcarbamylase (OTC) deficiency (X-linked). Presents with hyperammonaemia (>150 µmol/L), respiratory alkalosis, and encephalopathy. First-line: intravenous sodium benzoate (250 mg/kg) and sodium phenylbutyrate (250 mg/kg) as ammonia scavengers, plus arginine (200 mg/kg) to replenish intermediates. For NAGS deficiency, give N-carbamylglutamate (100 mg/kg). Avoid protein intake initially; use intravenous glucose and lipids.
• Tyrosinaemia Type I: Deficiency of fumarylacetoacetate hydrolase (FAH). Presents with failure to thrive, hepatomegaly, rickets, and acute porphyria-like crises. Pathognomonic: elevated succinylacetone in urine and blood. First-line: nitisinone (NTBC, 1 mg/kg/day) to inhibit 4-hydroxyphenylpyruvate dioxygenase. Also restrict tyrosine and phenylalanine intake. Without treatment, risk of hepatocellular carcinoma.
• Homocystinuria (Cystathionine Beta-Synthase Deficiency): Autosomal recessive; presents with ectopia lentis (downward lens dislocation), intellectual disability, thromboembolism, and marfanoid habitus. Labs: elevated plasma homocysteine and methionine. First-line: pyridoxine (vitamin B6, 50-500 mg/day) in pyridoxine-responsive forms; if unresponsive, add betaine (6-9 g/day) and restrict methionine. Screen for thromboembolic events.
• Glycogen Storage Disease Type I (von Gierke Disease): Deficiency of glucose-6-phosphatase. Presents in infancy with severe fasting hypoglycaemia, lactic acidosis, hyperuricaemia, and hypertriglyceridaemia. Key: hepatomegaly, doll-like facies. First-line: uncooked cornstarch (1-2 g/kg every 4-6 hours) to maintain normoglycaemia; avoid fructose and galactose. Monitor for gout and renal dysfunction.
• Methylmalonic Acidemia (MMA): Deficiency of methylmalonyl-CoA mutase or cobalamin cofactor defect. Presents with metabolic acidosis, hyperammonaemia, ketosis, and neutropenia. Labs: elevated methylmalonic acid in urine; normal B12 level. First-line: hydroxocobalamin (1 mg IM daily) for B12-responsive forms; a low-protein diet (restrict isoleucine, valine, methionine, threonine). Severe cases require haemodialysis. Check for long-term renal and neurological complications.
• Phenylketonuria (PKU): Deficiency of phenylalanine hydroxylase. Presents with microcephaly, developmental delay, fair skin, and musty odour. Labs: elevated phenylalanine (>1200 µmol/L) with normal tyrosine. First-line: phenylalanine-restricted diet (use special amino acid formula) started within first 2 weeks of life. For sapropterin-responsive cases, give BH4 (20 mg/kg/day). Monitor phenylalanine levels weekly; target 120-360 µmol/L in children.

Common Traps in Biochemistry Questions

• Assuming hyperammonaemia always indicates a urea cycle defect; consider organic acidemias (e.g., MMA, propionic acidemia) which also cause hyperammonaemia with metabolic acidosis.
• Confusing the treatment of MSUD with PKU: MSUD requires thiamine, not sapropterin; PKU uses BH4 only in responsive cases.
• Missing the respiratory alkalosis in UCDs as a clue; most metabolic encephalopathies cause acidosis, but UCDs cause alkalosis due to hyperventilation from ammonia toxicity.
• Forgetting that nitisinone elevates tyrosine levels; monitor for corneal crystals (tyrosine crystals) and adjust diet accordingly.
• Assuming all glycogen storage diseases cause hypoglycaemia; type II (Pompe) does not cause fasting hypoglycaemia but presents with cardiomyopathy and myopathy.
• Overlooking the need for haemodialysis in severe MSUD or MMA when encephalopathy is present; drug therapy alone is insufficient.

How to Revise Biochemistry for the PMDC NLE Step 1

Focus on neonatal metabolic crises: you will be given an infant with poor feeding, vomiting, lethargy, and abnormal labs. Prioritise interpreting the anion gap, ammonia, glucose, and urine organic acids. Memorise the first-line treatments for the top six disorders (MSUD, UCDs, tyrosinaemia, homocystinuria, von Gierke, MMA). Questions often present a scenario with a specific lab value (e.g., succinylacetone, methylmalonic acid) and ask for the enzyme defect or the immediate drug. Practise differentiating between disorders that cause hyperammonaemia with acidosis (organic acidemias) vs. alkalosis (UCDs). Also, know which vitamins are cofactors: thiamine (MSUD), B6 (homocystinuria), B12 (MMA), biotin (biotinidase deficiency). Do not waste time on detailed pathway intermediates; focus on diagnostic markers and management protocols.

Practise it: MedLumen has 50 Biochemistry questions for the PMDC NLE Step 1, each with a full explanation and references.