AIM LEARNING CYCLE · FINAL YEAR MBBS · KMU CURRICULUM · MS MODULE
Student Memory Support
TOPIC 7. Inherited Metabolic & Storage Disorders
High-yield recall, distinctions and clinical associations for revision
1. High-Yield Flashcards
Tap each question to reveal the answer.
What causes inherited metabolic disorders?
An inherited enzyme or transport defect causing toxic accumulation, product deficiency or failure of energy release.
Which test supports an acute hepatic porphyria during symptoms?
Spot urinary porphobilinogen (PBG), usually with aminolevulinic acid (ALA).
Which porphyria causes acute neurovisceral attacks without skin photosensitivity?
Acute intermittent porphyria (AIP).
Why can fasting trigger an acute hepatic porphyria attack?
It increases hepatic heme-precursor synthesis and precursor accumulation.
What is the specific treatment of a significant acute hepatic porphyria attack?
Intravenous hemin, alongside supportive care and trigger removal.
Which porphyria causes fragile blistering sun-exposed skin?
Porphyria cutanea tarda.
Which GSD presents with fasting hypoglycemia, hepatomegaly and lactic acidosis?
GSD I (von Gierke disease).
What distinguishes infantile Pompe disease clinically?
Hypotonia with hypertrophic cardiomyopathy from lysosomal glycogen accumulation.
What enzyme is deficient in Cori disease?
Glycogen debranching enzyme (GSD III).
What causes exercise-induced cramps and possible myoglobinuria in McArdle disease?
Deficiency of muscle glycogen phosphorylase (GSD V).
What accumulates in mucopolysaccharidoses (MPS)?
Glycosaminoglycans due to defective lysosomal breakdown.
Which MPS is X-linked and generally lacks corneal clouding?
MPS II (Hunter syndrome).
Which MPS has mainly skeletal involvement with relatively preserved intelligence?
MPS IV (Morquio syndrome).
What is an important dangerous neurological complication of MPS?
Cervical spinal cord compression.
Which enzyme deficiency causes classic galactosemia?
Galactose-1-phosphate uridyltransferase (GALT) deficiency.
Which serious infection is associated with neonatal classic galactosemia?
Escherichia coli sepsis.
What causes classic phenylketonuria (PKU)?
Phenylalanine hydroxylase deficiency, producing elevated phenylalanine.
Why must PKU treatment begin before symptoms appear?
Early phenylalanine restriction prevents irreversible neurodevelopmental injury.
2. Mnemonics
GSD Pattern Recognition
H–M–E
H = hepatic forms: fasting hypoglycemia and hepatomegaly; M = muscle forms: exercise cramps and intolerance; E = enzyme or molecular assay confirms subtype.
MPS Recognition
FACE
F = facial coarsening; A = airway problems; C = contractures/cardiac valve disease; E = enlarged liver and spleen. This is a recall aid, not diagnostic criteria.
Acute Porphyria
PAIN
P = painful abdominal attacks; A = autonomic signs (tachycardia/hypertension); I = impaired motor function (weakness); N = neurological symptoms. Confirm with urinary PBG.
3. Memory Tables
Major inherited metabolic disorders — quick differentiation
| Condition | Main clinical pattern | High-yield investigation |
|---|---|---|
| Acute intermittent porphyria | Neurovisceral attacks, no photosensitivity | Urine PBG during attack |
| Porphyria cutanea tarda | Fragile blistering sun-exposed skin | Porphyrin evaluation; assess iron overload |
| GSD I (von Gierke) | Fasting hypoglycemia, hepatomegaly, lactate ↑ | Glucose/lactate pattern; subtype confirmation |
| Pompe (GSD II) | Hypotonia and hypertrophic cardiomyopathy | Acid alpha-glucosidase activity |
| MPS | Coarse features, joint/skeletal changes, organomegaly | Urine GAG screen; enzyme confirmation |
| Galactosemia | Milk-associated neonatal jaundice, vomiting or sepsis | Erythrocyte GALT activity |
| PKU | High phenylalanine; later neurodevelopmental harm if untreated | Plasma phenylalanine |
4. Rapid Revision Points — Last-Minute Revision
- Acute porphyria = abdominal pain + autonomic or neurological features; test urine PBG during symptoms.
- AIP does not cause photosensitivity; some other porphyrias do.
- Fasting and porphyrinogenic drugs may trigger acute hepatic porphyria.
- Severe weakness during an acute porphyria attack can threaten ventilation.
- GSD I causes fasting hypoglycemia with hepatomegaly and elevated lactate.
- Pompe causes cardiomyopathy and hypotonia; specific enzyme replacement is available.
- MPS can cause airway obstruction and cervical spinal cord compression.
- Hunter syndrome is X-linked and generally does not cause corneal clouding.
- Milk-associated neonatal liver dysfunction suggests galactosemia; remove lactose/galactose urgently.
- Early phenylalanine control prevents brain injury; maternal PKU requires control before conception.
5. Clinical Memory Hooks
Severe episodic abdominal pain + neuropathy + hyponatremia → Acute hepatic porphyria → urinary PBG during attack
Infant with hepatomegaly + early-morning hypoglycemia → Hepatic glycogen storage disease → assess glucose/lactate
Infant hypotonia + hypertrophic cardiomyopathy → Pompe disease → acid alpha-glucosidase assay
Newborn illness after milk feeds + jaundice → Classic galactosemia → stop galactose/lactose and assess for sepsis
Normal-appearing newborn + raised screening phenylalanine → PKU → early dietary treatment before neurological injury
6. Starred High-Yield Exam Points
- ⭐ Spot urinary PBG during symptoms is the key initial test in suspected acute hepatic porphyria.
- ⭐ Intravenous hemin is specific treatment for significant acute hepatic porphyria attacks.
- ⭐ GSD I: hypoglycemia + hepatomegaly + lactic acidosis.
- ⭐ Pompe: hypotonia + hypertrophic cardiomyopathy; enzyme replacement targets the defect.
- ⭐ Classic galactosemia: GALT deficiency, milk-related neonatal illness and E. coli sepsis.
- ⭐ PKU: elevated phenylalanine; early treatment prevents major neurodevelopmental injury.
- ⭐ Maternal PKU: optimize phenylalanine before conception and through pregnancy.