NEET-PG Biochemistry Previous Year Questions (2017–2026)

Correct answer: C. Regan isoenzyme

Alkaline phosphatase isoenzymes differ in heat stability. The Regan isoenzyme is a placental-like (carcinoplacental) isoenzyme produced ectopically by tumours, classically bronchogenic carcinoma. Like true placental alkaline phosphatase it is the most heat-stable fraction, retaining activity after heating at 65 degrees Celsius. Bone alkaline phosphatase is the most heat-labile, and liver is intermediate, so surviving the heat test points to the Regan isoenzyme.

Incorrect Options:

• A. Bone isoenzyme: bone alkaline phosphatase is the most heat-labile fraction; it is destroyed by heating.
  
  Bone isoenzyme.
• B. Hepatic isoenzyme: the liver isoenzyme is only intermediate in heat stability, not heat-stable.
  
  Hepatic isoenzyme.
• D. Intestinal isoenzyme: the intestinal isoenzyme is moderately heat-stable but is not the tumour-associated Regan fraction.
  
  Intestinal isoenzyme.

High-Yield Facts:

• Heat stability order: placental/Regan > intestinal > liver > bone (bone most labile).
• The Regan isoenzyme is a carcinoplacental alkaline phosphatase, classically with bronchogenic carcinoma.
• Mnemonic: bone burns (labile); placenta persists (stable).
• Regan is identical to placental ALP; the Nagao variant is a related tumour isoenzyme.

Correct answer: D. Ornithine transcarbamylase

This is ornithine transcarbamylase (OTC) deficiency, the commonest urea cycle defect and the only X-linked one, so it hits male neonates. The block stops citrulline formation (hence low citrulline) and causes carbamoyl phosphate to accumulate and spill into the pyrimidine pathway, giving orotic aciduria. A normal haemogram separates it from hereditary orotic aciduria, in which megaloblastic anaemia is present.

Incorrect Options:

• A. Carbamoyl phosphate synthetase I: its deficiency also causes hyperammonaemia but with LOW orotic acid, since carbamoyl phosphate is not made.
  
  Carbamoyl phosphate synthetase I.
• B. Argininosuccinate synthetase: this causes citrullinaemia, with HIGH, not low, plasma citrulline.
  
  Argininosuccinate synthetase.
• C. Arginase: arginase deficiency causes argininaemia with spastic diplegia, not neonatal orotic aciduria.
  
  Arginase.

High-Yield Facts:

• OTC deficiency is the only X-linked urea cycle disorder and the most common.
• Low citrulline plus high orotic acid plus hyperammonaemia points to OTC deficiency.
• Normal haemogram distinguishes OTC deficiency from hereditary orotic aciduria (megaloblastic anaemia).
• CPS I deficiency gives hyperammonaemia but low orotic acid.

Correct answer: A. Autosomal dominant disorder

Advanced paternal age increases new point mutations because spermatogonia divide continuously and accumulate replication errors. Such de novo mutations typically produce autosomal dominant disorders that manifest in a single copy, the classic trio being achondroplasia, Apert syndrome and Marfan syndrome. In contrast, aneuploidy from non-disjunction is the maternal-age effect (for example Down syndrome).

Incorrect Options:

• B. Autosomal recessive disorders: these need two mutant alleles and are not driven by new paternal point mutations.
  
  Autosomal recessive disorders.
• C. X-linked disorders: these are not the principal effect of advanced paternal age.
  
  X-linked disorders.
• D. Aneuploidy: chromosomal non-disjunction is the maternal-age effect, not the paternal-age effect.
  
  Aneuploidy.

High-Yield Facts:

• Advanced paternal age raises de novo autosomal dominant point mutations.
• Classic examples: achondroplasia, Apert syndrome, Marfan syndrome.
• Advanced maternal age raises aneuploidy (non-disjunction), e.g. Down syndrome.
• Achondroplasia is caused by an activating FGFR3 mutation.