🧬 NRE Step 1 — Biochemistry
NRE Step 1 Biochemistry High-Yield Topics 2026 — Pathways & Clinical Correlations
Metabolic pathways, enzyme deficiencies, vitamins, and lab value interpretation — all connected to the clinical scenarios NRE actually tests.
A 6-month-old infant is brought to the clinic with poor feeding, vomiting after milk feeds, and jaundice. Urine is positive for reducing substances but negative for glucose on dipstick. The mother says the symptoms started when breastfeeding was replaced with formula milk. What is the diagnosis, what enzyme is deficient, and what is the dietary treatment?
This is how biochemistry appears in NRE Step 1. Not as a pathway diagram to memorize. Not as a question asking you to name all the enzymes of the citric acid cycle. It presents a patient — usually a child — with a metabolic crisis triggered by a dietary exposure, and tests whether you can connect the clinical picture to the underlying enzyme defect and its treatment.
Graduates who approach biochemistry as pure memorization dread these questions. Those who learn biochemistry as a set of clinically anchored stories — what happens when this enzyme is missing, what the patient looks like, what makes it worse, what fixes it — score reliably on every biochemistry MCQ in NRE.
📌 Biochemistry Weightage in NRE Step 1
Biochemistry contributes approximately 5–9% of NRE Step 1 direct questions, with additional biochemistry concepts embedded in clinical medicine (DKA acid-base, liver function tests), paediatrics (inborn errors of metabolism), and nutrition questions. The highest-yield areas are: inborn errors of metabolism, vitamins and deficiencies, acid-base interpretation, and clinically applied enzyme markers.
1. Carbohydrate Metabolism — Clinical Correlations
NRE does not test the full glycolysis or gluconeogenesis pathway step-by-step. It tests what happens clinically when a key part of carbohydrate metabolism fails — in a specific patient, in a specific dietary context.
Glycolysis & Gluconeogenesis — The Clinical Anchor Points
🔑 The Fasting State — NRE Tests This Logic
After an overnight fast: glycogen stores depleted → gluconeogenesis from amino acids (alanine, glutamine) and glycerol takes over. The liver is the primary site. In liver failure, gluconeogenesis fails → hypoglycaemia. This is why cirrhotic patients are fasting-hypoglycaemic. NRE tests this as both a biochemistry and a clinical medicine question.
Inborn Errors of Carbohydrate Metabolism
| Condition | Deficient Enzyme | Clinical Picture | Key Diagnostic Feature | Treatment | Frequency |
|---|---|---|---|---|---|
| Galactosaemia | Galactose-1-phosphate uridyltransferase (classic type) | Neonate: jaundice, vomiting, poor feeding after milk. Cataracts, hepatomegaly, E. coli sepsis in neonates. Long-term: intellectual disability. | Reducing substances in urine (Clinitest +ve) but glucose dipstick NEGATIVE — distinguishes from glucose in urine | Galactose-free diet (no dairy). Lactose-free formula. | Very High |
| Glucose-6-phosphate dehydrogenase (G6PD) deficiency | G6PD (protects RBCs from oxidative damage) | Haemolytic anaemia triggered by: primaquine, dapsone, nitrofurantoin, sulphonamides, fava beans, infection. Heinz bodies + bite cells on film. | G6PD enzyme assay (best done after haemolytic episode resolves — levels falsely normal during crisis as deficient cells already lysed) | Avoid triggers. Supportive during crisis. Folic acid. | Very High |
| Glycogen Storage Diseases — Von Gierke (Type I) | Glucose-6-phosphatase | Severe fasting hypoglycaemia, hepatomegaly, lactic acidosis, hyperuricaemia (gout), hyperlipidaemia. “Doll-like face,” protuberant abdomen. | Glucagon challenge: no rise in blood glucose (gluconeogenesis blocked) | Frequent feeding; uncooked cornstarch; avoid fasting | High |
| Pompe Disease (Type II) | Acid maltase (α-glucosidase) | Infantile: hypotonia (“floppy baby”), cardiomegaly, respiratory failure. Adult: proximal myopathy, respiratory failure. | Enzyme assay; muscle biopsy: PAS+ vacuoles (glycogen accumulation) | Enzyme replacement therapy (alglucosidase alfa) | High |
| Lactase deficiency | Lactase (brush border enzyme) | Bloating, flatulence, osmotic diarrhoea after dairy ingestion. Common in adults of African and Asian descent (primary); secondary to Crohn’s, coeliac, acute gastroenteritis. | Hydrogen breath test (gold standard); lactose tolerance test; stool pH <5.5 | Lactose-free diet; lactase enzyme supplements | High |
| Fructose intolerance (hereditary) | Aldolase B (fructose-1-phosphate aldolase) | Vomiting, hypoglycaemia, liver damage after ingestion of fructose, sucrose, or sorbitol. Symptoms start when weaned (fruit, table sugar introduced). | Fructose load test (avoid in acute illness); genetic testing | Fructose-free, sucrose-free diet | Moderate |
2. Amino Acid & Protein Metabolism — Inborn Errors
Each inborn error of amino acid metabolism has a distinct clinical fingerprint — a specific body fluid odour, a specific urine finding, or a specific neurological picture. NRE uses these fingerprints to ask you to identify the condition from a clinical description.
| Condition | Deficient Enzyme | Amino Acid Accumulates | Clinical Fingerprint | Treatment | Frequency |
|---|---|---|---|---|---|
| Phenylketonuria (PKU) | Phenylalanine hydroxylase | Phenylalanine → phenylpyruvate (excreted in urine) | Normal at birth (screened by Guthrie test). Progressive intellectual disability, seizures, eczema, mousy/musty urine odour. Fair skin and hair (melanin synthesis reduced). Maternal PKU: if mother untreated during pregnancy → microcephaly, cardiac defects in infant. | Phenylalanine-restricted diet (lifelong). Sapropterin (BH4 cofactor) for some variants. | Very High |
| Maple Syrup Urine Disease (MSUD) | Branched-chain α-ketoacid dehydrogenase | Leucine, isoleucine, valine (branched-chain amino acids) | Maple syrup odour of urine. Neonatal encephalopathy (poor feeding, lethargy, seizures) in first week of life. Hypoglycaemia. Leucine most neurotoxic. | Branched-chain amino acid–restricted diet; thiamine (cofactor) in responsive variants; dialysis in acute crisis | High |
| Alkaptonuria | Homogentisate oxidase | Homogentisic acid (tyrosine pathway) | Urine turns black on standing (oxidation of homogentisic acid). Ochronosis (dark pigment in connective tissue — cartilage, sclera, earwax). Arthritis in adulthood. | Low protein diet (limited benefit); nitisinone (reduces homogentisic acid production) | High |
| Homocystinuria | Cystathionine synthase (most common) OR MTHFR deficiency | Homocysteine | Marfanoid habitus (tall, lens dislocation — DOWNWARD in homocystinuria vs UPWARD in Marfan syndrome). Intellectual disability, thromboembolic events, premature atherosclerosis. Lens dislocation direction is a classic NRE MCQ trap. | Pyridoxine (B6 — cofactor, works in ~50%); methionine-restricted diet + cysteine supplement; folate + B12 if MTHFR type | Very High |
| Cystinuria | Cystine transporter defect (not an enzyme deficiency — a transport defect) | Cystine (also ornithine, lysine, arginine) | Recurrent kidney stones (cystine stones — radiopaque but less dense than calcium). Hexagonal crystals in urine microscopy. Young patient with recurrent stones. | High fluid intake; urinary alkalinisation (potassium citrate); penicillamine (chelates cystine) | High |
| Urea Cycle Defects (e.g., OTC deficiency) | Ornithine transcarbamylase (X-linked — most common urea cycle defect) | Ammonia (hyperammonaemia) | Neonatal encephalopathy: lethargy → coma after protein feeding. Hyperammonaemia + respiratory alkalosis (ammonia stimulates hyperventilation). No metabolic acidosis (unlike organic acidaemias). | Protein restriction; sodium benzoate/phenylbutyrate (alternative nitrogen excretion); liver transplant (definitive) | High |
🔑 Lens Dislocation Direction — The NRE Trap
This comparison appears directly in NRE MCQs: Marfan syndrome → lens dislocates UPWARD (superotemporal) — fibrillin-1 defect weakens superior zonule. Homocystinuria → lens dislocates DOWNWARD (inferonasal) — homocysteine damages inferior zonule. Both conditions produce a tall patient with long limbs and lens dislocation — the direction distinguishes them.
3. Lipid Metabolism & Lipid Disorders
Lipoprotein Classification — Clinical Application
| Lipoprotein | Primary Cargo | Clinical Relevance in NRE |
|---|---|---|
| Chylomicrons | Dietary triglycerides (exogenous) | Deficiency → lipoprotein lipase deficiency: eruptive xanthomas, acute pancreatitis, creamy plasma. Abetalipoproteinaemia: no chylomicrons or VLDL → fat-soluble vitamin deficiency (A, D, E, K), acanthocytosis, retinitis pigmentosa. |
| VLDL | Endogenous triglycerides (liver-derived) | High TG → hypertriglyceridaemia → risk of pancreatitis. Metabolised to IDL → LDL. |
| LDL | Cholesterol (to peripheral tissues) | Familial hypercholesterolaemia: LDL receptor defect → very high LDL → premature atherosclerosis, tendon xanthomas, corneal arcus in young patients. Statins reduce LDL by upregulating LDL receptors. |
| HDL | Reverse cholesterol transport | High HDL = protective. Low HDL = independent CVD risk factor. Exercise and moderate alcohol raise HDL (NRE tests this concept). |
Storage Disorders — Lipid Accumulation Diseases
Lysosomal Storage
Gaucher Disease
- Glucocerebrosidase deficiency
- Glucocerebroside accumulates
- Hepatosplenomegaly, bone pain, pancytopaenia
- “Crinkled tissue paper” macrophages
- Most common lysosomal storage disease
- Tx: enzyme replacement (imiglucerase)
Lysosomal Storage
Niemann-Pick Disease
- Sphingomyelinase deficiency (Type A/B)
- Sphingomyelin accumulates
- Hepatosplenomegaly, progressive neurodegeneration
- Cherry-red spot on macula (Type A)
- “Foamy” lipid-laden macrophages
- No effective enzyme therapy for Type A
Lysosomal Storage
Tay-Sachs Disease
- Hexosaminidase A deficiency
- GM2 ganglioside accumulates
- Progressive neurodegeneration
- Cherry-red spot on macula
- NO hepatosplenomegaly (vs Niemann-Pick)
- Ashkenazi Jewish population (higher incidence)
Lysosomal Storage
Fabry Disease
- Alpha-galactosidase A deficiency
- X-linked — males severely affected
- Painful crises in extremities
- Angiokeratomas (skin)
- Renal failure, cardiomyopathy
- Tx: enzyme replacement (agalsidase)
4. Vitamins — Deficiency & Toxicity
Vitamin questions in NRE are almost always clinical: a patient with a specific constellation of symptoms that points to a single vitamin deficiency. Learn each deficiency as a clinical picture, not a list of functions.
Fat-Soluble Vitamins (A, D, E, K)
| Vitamin | Deficiency Syndrome | Toxicity | NRE High-Yield Point | Frequency |
|---|---|---|---|---|
| Vitamin A | Night blindness (first sign), xerophthalmia, Bitot’s spots, keratomalacia → corneal ulceration → blindness. Follicular hyperkeratosis (toad skin). | Pseudotumour cerebri (raised ICP), hepatotoxicity, teratogenicity (isotretinoin). Carotenodermia from excess carrots (NOT toxic — skin turns yellow, sclera spared). | WHO recommends Vitamin A supplementation in measles in malnourished children → reduces mortality. Also used in NRE community health questions. | Very High |
| Vitamin D | Rickets (children): bow legs, rachitic rosary, craniotabes, Harrison’s sulcus, delayed fontanelle closure. Osteomalacia (adults): bone pain, proximal myopathy, Looser’s zones on X-ray. | Hypercalcaemia, hypercalciuria, nephrocalcinosis, metastatic calcification. | 25-OH vitamin D = best test for body stores. 1,25-OH vitamin D (calcitriol) = active form. PTH rises in Vit D deficiency (secondary hyperparathyroidism). NRE tests calcium-phosphate-PTH-Vit D axis as an integrated question. | Very High |
| Vitamin E | Haemolytic anaemia (neonates, premature infants), spinocerebellar ataxia, peripheral neuropathy, acanthocytosis in abetalipoproteinaemia. | Rare; may interfere with Vitamin K (potentiates warfarin effect). | Vitamin E is the antioxidant that protects cell membranes from lipid peroxidation. Deficiency in abetalipoproteinaemia is a classic NRE combined question (lipid + vitamin). | Moderate |
| Vitamin K | Bleeding tendency: prolonged PT (extrinsic pathway affected first). Neonatal haemorrhagic disease (breastfed infants — breast milk low in Vit K). | High doses of synthetic Vitamin K (menadione) → haemolytic anaemia in neonates. | Warfarin works by inhibiting Vitamin K–dependent clotting factors (II, VII, IX, X) and proteins C and S. Vitamin K reverses warfarin — NRE tests urgency: fresh frozen plasma (immediate) vs Vitamin K (slower). PT/INR specifically prolonged. | Very High |
Water-Soluble Vitamins (B-complex & C)
| Vitamin | Deficiency Syndrome & Clinical Picture | At-Risk Population | Frequency |
|---|---|---|---|
| B1 (Thiamine) | Dry beriberi: peripheral neuropathy. Wet beriberi: high-output cardiac failure, oedema. Wernicke’s encephalopathy: confusion + ataxia + ophthalmoplegia (triad). Korsakoff’s psychosis: anterograde amnesia, confabulation (irreversible if B1 not given promptly). | Alcoholics (#1), chronic IV dextrose without thiamine, hyperemesis gravidarum, malnutrition | Very High |
| B2 (Riboflavin) | Cheilosis (fissures at mouth corners), angular stomatitis, glossitis, corneal vascularisation, photophobia, normocytic anaemia. | Malnutrition; rarely isolated | Moderate |
| B3 (Niacin) | Pellagra: 4 Ds — Diarrhoea + Dermatitis (sun-exposed areas, Casal’s necklace) + Dementia + Death. Carcinoid syndrome → pellagra (tryptophan diverted to serotonin). | Maize-based diets (maize low in niacin + tryptophan); carcinoid tumour; Hartnup disease (tryptophan absorption defect) | Very High |
| B6 (Pyridoxine) | Peripheral neuropathy (identical to isoniazid toxicity — isoniazid is a B6 antagonist). Sideroblastic anaemia. Seborrhoeic dermatitis, glossitis. | Isoniazid therapy (always give B6 with INH). Alcoholism. OCP use. | Very High |
| B9 (Folate) | Megaloblastic anaemia (macrocytic + hypersegmented neutrophils). Neural tube defects (if deficient in periconceptional period). NO neurological symptoms (unlike B12). | Pregnancy (supplementation mandatory), methotrexate use (folate antagonist), malnutrition, alcohol, phenytoin | Very High |
| B12 (Cobalamin) | Megaloblastic anaemia + subacute combined degeneration of spinal cord (posterior + lateral columns → ataxia, proprioception loss, spastic paraparesis). Glossitis. Dementia. Neurological symptoms ABSENT in folate deficiency — key differentiator. | Pernicious anaemia (anti-intrinsic factor antibodies), strict vegans, gastrectomy/ileal resection, Crohn’s (terminal ileum), fish tapeworm (Diphyllobothrium) | Very High |
| Vitamin C (Ascorbic acid) | Scurvy: perifollicular haemorrhages, corkscrew hairs, bleeding gums, poor wound healing, haemarthrosis. “Scurvy → sailors without citrus.” Impaired collagen synthesis (hydroxylation of proline and lysine requires Vit C). | Elderly living alone on poor diet, alcoholics, infants on unfortified formula | High |
⚠️ B12 vs Folate — The NRE Differentiator That Saves Marks
Both cause identical megaloblastic anaemia on blood film. The NRE differentiates them by one critical feature: only B12 deficiency causes neurological symptoms (subacute combined degeneration). Giving folate to a B12-deficient patient corrects the anaemia but allows the neurological damage to progress silently — a classic dangerous clinical error that NRE tests as a management MCQ. Always check B12 before starting folate supplementation.
5. Acid-Base Interpretation — The Framework NRE Uses
Arterial blood gas (ABG) interpretation appears in NRE Step 1 consistently, embedded in clinical scenarios (DKA, COPD exacerbation, vomiting, diarrhoea, salicylate overdose). You need a systematic approach that works in 60 seconds per question.
The 4-Step ABG Approach
Step 1Check pH
Step 2Primary disorder (PaCO₂ or HCO₃⁻)?
Step 3Compensation?
Step 4Clinical context
| Disorder | pH | PaCO₂ | HCO₃⁻ | Classic NRE Clinical Scenario |
|---|---|---|---|---|
| Respiratory acidosis | ↓ | ↑ (primary) | ↑ (compensation) | COPD exacerbation, opioid overdose, hypoventilation, neuromuscular disease (GBS, MG) |
| Respiratory alkalosis | ↑ | ↓ (primary) | ↓ (compensation) | Anxiety/hyperventilation, high altitude, salicylate toxicity (early — respiratory stimulation), pregnancy, PE |
| Metabolic acidosis | ↓ | ↓ (compensation) | ↓ (primary) | DKA, lactic acidosis, renal failure, diarrhoea (loss of HCO₃⁻), methanol/ethylene glycol poisoning |
| Metabolic alkalosis | ↑ | ↑ (compensation) | ↑ (primary) | Vomiting (loss of HCl), pyloric stenosis, excessive diuretics (loop/thiazide — K⁺ and H⁺ loss), Conn’s syndrome |
🔑 Anion Gap — Narrows the Metabolic Acidosis Differential
Anion Gap = Na⁺ − (Cl⁻ + HCO₃⁻). Normal = 8–12 mEq/L.High anion gap metabolic acidosis (MUDPILES): Methanol | Uraemia | DKA | Propylene glycol | Isoniazid/Iron | Lactic acidosis | Ethylene glycol | Salicylates.
Normal anion gap (hyperchloraemic) metabolic acidosis: Diarrhoea (loss of HCO₃⁻), RTA (renal tubular acidosis), Addison’s disease, acetazolamide. NRE tests anion gap as a question stem: “metabolic acidosis with a high anion gap” → DKA, lactic acidosis, or poisoning.
6. Clinically Applied Enzyme Markers
NRE uses enzyme and biomarker results as part of clinical scenarios. Know which marker rises in which condition, its timing, and what it means for management.
| Marker | Condition | NRE-Tested Detail |
|---|---|---|
| Troponin I/T | Myocardial infarction | Rises 3–6 hrs after MI, peaks 12–24 hrs, returns to normal in 7–10 days. High sensitivity troponin can detect at 1 hr (hs-cTn). Most sensitive and specific marker for myocardial injury. |
| CK-MB | MI (less specific than troponin) | Rises 4–6 hrs, peaks 12–24 hrs, returns to normal in 48–72 hrs. Useful for detecting reinfarction (returns to normal faster than troponin). |
| AST / ALT | Liver disease | ALT more liver-specific. AST:ALT ratio >2:1 suggests alcoholic liver disease (ALD). Both raised in viral hepatitis, NASH, drug-induced liver injury. |
| GGT | Alcohol use, liver disease, cholestasis | Most sensitive marker of alcohol intake. Raised in cholestasis (with ALP). Used to confirm liver origin of raised ALP. |
| ALP | Cholestasis, bone disease, pregnancy | Raised in: biliary obstruction, Paget’s disease, bone metastases, rickets, pregnancy. GGT raised with ALP = liver cause. GGT normal with ALP = bone or placental cause. |
| Amylase / Lipase | Pancreatitis | Lipase more sensitive and specific than amylase (stays elevated longer). Amylase also raised in: parotitis, perforated peptic ulcer, intestinal ischaemia. Level does not predict severity. |
| LDH | Haemolysis, MI, PCP pneumonia, malignancy | Non-specific but high LDH in PCP pneumonia is a diagnostic clue. High LDH in haemolysis (with low haptoglobin, raised unconjugated bilirubin, reticulocytosis). |
| PSA | Prostate disease | Raised in: prostate cancer, BPH, prostatitis, after DRE or catheterisation. Not diagnostic alone — used for screening and monitoring. Age-specific reference ranges. |
| BNP / NT-proBNP | Heart failure | Rises with ventricular wall stress. High BNP + dyspnoea = heart failure until proven otherwise. Normal BNP has high negative predictive value — helps rule out cardiac cause of dyspnoea in ED. |
What You Can Safely Skip in Biochemistry
- Full citric acid cycle step-by-step. Knowing that the TCA cycle occurs in the mitochondria and generates NADH/FADH₂ for oxidative phosphorylation is sufficient. You will not be asked to name all 8 enzymes in sequence.
- Electron transport chain complex numbering. The clinical correlation (Complex I deficiency → Leigh syndrome; cyanide blocks Complex IV) is more likely than memorizing which complex does what.
- Detailed fatty acid oxidation steps. Know that beta-oxidation occurs in mitochondria, requires carnitine shuttle (blocked by pivalate; carnitine deficiency → myopathy + hypoglycaemia), and produces acetyl-CoA. The step-by-step enzymes are not NRE-level content.
- Purine/pyrimidine synthesis pathway in detail. Lesch-Nyhan syndrome (HGPRT deficiency → gout + self-mutilation + neurological deficits) and the drugs that inhibit purine synthesis (methotrexate, allopurinol mechanism) are tested. The full synthetic pathway is not.
Frequently Asked Questions
How deeply is biochemistry tested in NRE Step 1?
NRE tests biochemistry at the clinical application level — not at the research or detailed pathway level. The pattern is consistent: an infant or child with a metabolic crisis, a vitamin deficiency presented as a cluster of symptoms, or an ABG that needs interpretation in the context of a clinical condition. You are never asked to draw a pathway from memory or name all enzymes in a cycle. You are asked to connect a biochemical defect to its clinical consequence and treatment. This guide covers everything at the level NRE tests.
Are inborn errors of metabolism always paediatric questions in NRE?
Mostly yes — inborn errors of metabolism typically present in infancy or early childhood, which is why they appear in NRE as paediatric clinical scenarios. However, some conditions present in adulthood: Fabry disease (renal failure, cardiomyopathy in 30s–40s), alkaptonuria (arthritis in adulthood), Wilson’s disease (young adults), and adult-onset Pompe disease (proximal myopathy). When NRE presents an inborn error in an adult, it is usually because the diagnosis was missed in childhood or the condition has a late-onset form.
How do I approach ABG questions in NRE if I find them confusing?
Use the 4-step framework every time, without exception: (1) Is the pH acidotic or alkalotic? (2) Which primary parameter is abnormal in the same direction — PaCO₂ (respiratory) or HCO₃⁻ (metabolic)? (3) Is there compensation — is the other parameter moving in the same direction to try to normalise pH? (4) Does the clinical scenario fit? If you apply this sequence to 30–40 practice ABGs before your exam, the approach becomes automatic and ABG questions become reliable marks rather than guesswork. We practise this in our NRE Step 1 live sessions using clinical case ABGs.
PKU is screened at birth — why would NRE ask about it?
NRE tests PKU from multiple angles: (1) diagnosis — what the Guthrie test screens for and when it should be done (after 48–72 hrs of milk feeding, not at birth before protein intake); (2) maternal PKU — an uncontrolled PKU mother who becomes pregnant has high phenylalanine levels that damage the fetus even if the fetus does not have PKU (microcephaly, cardiac defects); (3) dietary management — the phenylalanine-restricted diet must be lifelong, and relaxation in adulthood leads to neurological deterioration. These three angles each generate separate NRE question formats from the same condition.
Finding Biochemistry the Hardest Part of NRE Prep?
You are not alone. Most China-trained graduates find biochemistry harder in NRE format than in university exams. Our live sessions teach every inborn error, vitamin deficiency, and ABG through clinical cases — the same way NRE asks them. Message us to get started.
💬 Message on WhatsApp