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Amino Acid Disorders - NEET PG Biochemistry

Feb 10, 2023

amino acid disorders

Amino acid disorders are important topics for NEET PG because they are a group of inherited genetic disorders that affect the metabolism of amino acids, leading to a variety of health problems. These disorders are significant because they are potentially life-threatening and can lead to intellectual disability, developmental delays, and other serious health issues. Knowledge of amino acid disorders is crucial for NEET PG exam preparation

Read this blog further and get an overview of this important biochemistry topic.

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Glycine disorders



Nonketotic hyperglycinemia

Glycine transaminase deficient/defective

  • C/f

↑ oxalate in urine 

extra renal oxalate deposits

Oxalate stones

Glycine transporter defect

  • C/f
    • ↑ glycine in urine 
    • Urine oxalate normal
    • Risk of oxalate stones

Glycine cleavage system defect

  • C/f
    • ↑ glycine in serum, urine and CSF
    • Mental retardation
    • Seizures

Phenylketonuria (PKU)

  • Enzyme deficient: Phenylalanine hydroxylase
amino acid disorders

[Phenyl-keto-uria → phenyl- keto acid (phenyl Pyruvate) - in urine]

  • C/F
    • Mousy/musty body odor (due to phenylacetate)
    • Severe MR (due to Phenylalanine)
  • Diagnosis
    • Fecl3 urine test → Green color (d/t Phenyl pyruvate)
  • Rx
    1. Lifelong Restriction of phenylalanine in diet 
      • Aspartame (artificial sweetener) is contraindicated as aspartame is made of Aspartate + Phenylalanine
    2. Give tyrosine & tryptophan
    3. Tetra hydro Biopterin Supplementation helps in some patients

Also Read : An easy approach to controversial questions in Biochemistry

Maple Syrup Urine Disease (MSUD)

  • Enzyme deficient: Branched Chain Keto Acid (BCKA) dehydrogenase or decarboxylase
  • Accumulation of Branched chain amino acids and keto acids occurs
  • Urine odor is of Burnt sugar / Maple syrup odor
  • Clinical features
    • Mental Retardation
    • Ketosis
    • If untreated, coma & death can occur
    • Has high mortality rate 
  • Rx
    1. Vitamin B1 supplementation (coenzyme for BCKA dehydrogenase)
    2. dietary restriction of branched chain amino acids

Also Read : Important Topics in Biochemistry for AIIMS-PG by Dr. Smily Pruthi

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Alkaptonuria/Black bone disease

  • Enzyme defect: Homogentisate dioxygenase (previously k/a Homogentisate oxidase)
  • Homogentisic acid accumulates
  • C/F
    • On standing color of urine turns black due to oxidation of Homogentisic acid
    • Age of onset: 30 - 40 years
    • no MR
    • Homogentisic acid gets polymerized in to alkapton bodies which accumulates in:
      • Cartilages (Nose, Ear Pinna, IV discs): Arthritis
      • Connective tissue: Bluish black color
      • Intervertebral discs: lower back pain (due to 
  • Collectively, accumulation of homogentisic acid in all body parts is termed as Ochronosis and arthritis as Ochronotic arthritis.

Also Read : Biochemistry FAQs answered by Dr. Smily Pruthi


  • Enzyme deficient: Tyrosinase (makes melanin from tyrosine)
  • Tyrosine is an oxidase enzyme & requires copper for its action. 
  • c/f
  • Hypopigmentation (milky white skin, Blonde hair, and Red eye color)

Important Information


  • Patchy hypopigmentation
  • Tyrosinase is normal 

Also Read: Important Topics in Biochemistry for NEET-PG by Dr. Smily Pruthi

How to Prepare Biochemistry for Medical PG Entrance | PrepLadder

Homocystinuria (HCU)

  • Two types:
    • Acquired
      • Due to vitamin B6, B9 and B12 deficiency
    • Genetic
      • Due to Enzyme Cystathionine beta Synthase defect 
      • Here cysteine becomes essential.
      • c/f
        • ↑ Serum Homocysteine (has SH - group)
        • ↑ urine Homocystine (made of two homocysteine by S-S bond)

Cyanide nitroprusside test is positive.

Important Information 
How to distinguish between vit B6, B9 and B12 def

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