COMSAE Phase 1 Form 113 Practice Exam 2026
| 200 MCQs with Answers & Explanations
Most Covered Areas:
Biochemistry & Genetics
Microbiology & Immunology
Pharmacology
Cardiovascular & Respiratory
Renal, GI & Endocrine
Neurology & Musculoskeletal
Hematology & Oncology
Osteopathic Principles & OMT
Epidemiology & Biostatistics
Clinical Case-Based Questions
Section 1: Biochemistry, Genetics, and Cellular Biology (Questions 1–20)
1. A patient has a deficiency of glucose-6-phosphate dehydrogenase (G6PD). Which
mechanism best explains the resulting red blood cell injury?
,A. Reduced synthesis of hemoglobin B. Impaired regeneration of reduced glutathione C.
Inhibition of mitochondrial oxidative phosphorylation D. Increased production of intrinsic
factor
Correct answer: B. Impaired regeneration of reduced glutathione
Explanation: G6PD generates NADPH through the pentose phosphate pathway. NADPH
maintains reduced glutathione, which protects red blood cells from oxidative damage.
Deficiency can cause hemolysis after oxidative stress.
2. Which enzyme is deficient in classic phenylketonuria (PKU)?
A. Homogentisate oxidase B. Branched-chain alpha-ketoacid dehydrogenase C.
Phenylalanine hydroxylase D. Galactose-1-phosphate uridyltransferase
Correct answer: C. Phenylalanine hydroxylase
Explanation: Phenylalanine hydroxylase converts phenylalanine into tyrosine. Deficiency
causes phenylalanine accumulation and can lead to intellectual disability, seizures, and
hypopigmentation if untreated.
3. A child develops vomiting, hypoglycemia, and liver dysfunction after consuming
fruit. Which enzyme deficiency is most likely?
A. Aldolase B B. Lactase C. Glucose-6-phosphatase D. Pyruvate dehydrogenase
Correct answer: A. Aldolase B
Explanation: Hereditary fructose intolerance results from aldolase B deficiency. Fructose-1-
phosphate accumulates, trapping phosphate and inhibiting gluconeogenesis and
glycogenolysis.
,4. Which vitamin deficiency impairs the conversion of methylmalonyl-CoA to
succinyl-CoA?
A. Vitamin B1 B. Vitamin B3 C. Vitamin B6 D. Vitamin B12
Correct answer: D. Vitamin B12
Explanation: Vitamin B12 is a cofactor for methylmalonyl-CoA mutase. Deficiency increases
methylmalonic acid and can cause megaloblastic anemia and neurological abnormalities.
5. Which enzyme is directly inhibited by methotrexate?
A. Thymidylate synthase B. Dihydrofolate reductase C. Xanthine oxidase D. HMG-CoA
reductase
Correct answer: B. Dihydrofolate reductase
Explanation: Methotrexate inhibits dihydrofolate reductase, reducing tetrahydrofolate
availability and impairing nucleotide synthesis. This preferentially affects rapidly dividing
cells.
6. A patient with chronic alcohol use develops confusion, ataxia, and
ophthalmoplegia. Which vitamin deficiency is most likely?
A. Vitamin C B. Vitamin D C. Thiamine (B1) D. Vitamin K
Correct answer: C. Thiamine (B1)
Explanation: Thiamine is required by enzymes including pyruvate dehydrogenase and
alpha-ketoglutarate dehydrogenase. Deficiency can cause Wernicke encephalopathy,
characterized by confusion, ataxia, and ocular abnormalities.
, 7. Which enzyme is rate-limiting in cholesterol synthesis?
A. HMG-CoA reductase B. Lipoprotein lipase C. Acetyl-CoA carboxylase D. Hormone-
sensitive lipase
Correct answer: A. HMG-CoA reductase
Explanation: HMG-CoA reductase converts HMG-CoA into mevalonate. Statins inhibit this
enzyme, reducing hepatic cholesterol synthesis and increasing LDL receptor expression.
8. Which biochemical pathway generates ribose-5-phosphate and NADPH?
A. Urea cycle B. Glycolysis C. Beta-oxidation D. Pentose phosphate pathway
Correct answer: D. Pentose phosphate pathway
Explanation: The pentose phosphate pathway generates NADPH for reductive biosynthesis
and antioxidant defense, as well as ribose-5-phosphate for nucleotide synthesis.
9. A patient has elevated lactate following severe tissue hypoxia. Why does lactate
increase?
A. Increased aerobic oxidative phosphorylation B. Increased conversion of pyruvate to
lactate to regenerate NAD+ C. Inhibition of all glycolysis D. Increased conversion of lactate
into ketones in muscle
Correct answer: B. Increased conversion of pyruvate to lactate to regenerate NAD+
Explanation: Lactate dehydrogenase converts pyruvate into lactate while regenerating
NAD+, allowing glycolysis to continue producing ATP when oxidative metabolism is
inadequate.
| 200 MCQs with Answers & Explanations
Most Covered Areas:
Biochemistry & Genetics
Microbiology & Immunology
Pharmacology
Cardiovascular & Respiratory
Renal, GI & Endocrine
Neurology & Musculoskeletal
Hematology & Oncology
Osteopathic Principles & OMT
Epidemiology & Biostatistics
Clinical Case-Based Questions
Section 1: Biochemistry, Genetics, and Cellular Biology (Questions 1–20)
1. A patient has a deficiency of glucose-6-phosphate dehydrogenase (G6PD). Which
mechanism best explains the resulting red blood cell injury?
,A. Reduced synthesis of hemoglobin B. Impaired regeneration of reduced glutathione C.
Inhibition of mitochondrial oxidative phosphorylation D. Increased production of intrinsic
factor
Correct answer: B. Impaired regeneration of reduced glutathione
Explanation: G6PD generates NADPH through the pentose phosphate pathway. NADPH
maintains reduced glutathione, which protects red blood cells from oxidative damage.
Deficiency can cause hemolysis after oxidative stress.
2. Which enzyme is deficient in classic phenylketonuria (PKU)?
A. Homogentisate oxidase B. Branched-chain alpha-ketoacid dehydrogenase C.
Phenylalanine hydroxylase D. Galactose-1-phosphate uridyltransferase
Correct answer: C. Phenylalanine hydroxylase
Explanation: Phenylalanine hydroxylase converts phenylalanine into tyrosine. Deficiency
causes phenylalanine accumulation and can lead to intellectual disability, seizures, and
hypopigmentation if untreated.
3. A child develops vomiting, hypoglycemia, and liver dysfunction after consuming
fruit. Which enzyme deficiency is most likely?
A. Aldolase B B. Lactase C. Glucose-6-phosphatase D. Pyruvate dehydrogenase
Correct answer: A. Aldolase B
Explanation: Hereditary fructose intolerance results from aldolase B deficiency. Fructose-1-
phosphate accumulates, trapping phosphate and inhibiting gluconeogenesis and
glycogenolysis.
,4. Which vitamin deficiency impairs the conversion of methylmalonyl-CoA to
succinyl-CoA?
A. Vitamin B1 B. Vitamin B3 C. Vitamin B6 D. Vitamin B12
Correct answer: D. Vitamin B12
Explanation: Vitamin B12 is a cofactor for methylmalonyl-CoA mutase. Deficiency increases
methylmalonic acid and can cause megaloblastic anemia and neurological abnormalities.
5. Which enzyme is directly inhibited by methotrexate?
A. Thymidylate synthase B. Dihydrofolate reductase C. Xanthine oxidase D. HMG-CoA
reductase
Correct answer: B. Dihydrofolate reductase
Explanation: Methotrexate inhibits dihydrofolate reductase, reducing tetrahydrofolate
availability and impairing nucleotide synthesis. This preferentially affects rapidly dividing
cells.
6. A patient with chronic alcohol use develops confusion, ataxia, and
ophthalmoplegia. Which vitamin deficiency is most likely?
A. Vitamin C B. Vitamin D C. Thiamine (B1) D. Vitamin K
Correct answer: C. Thiamine (B1)
Explanation: Thiamine is required by enzymes including pyruvate dehydrogenase and
alpha-ketoglutarate dehydrogenase. Deficiency can cause Wernicke encephalopathy,
characterized by confusion, ataxia, and ocular abnormalities.
, 7. Which enzyme is rate-limiting in cholesterol synthesis?
A. HMG-CoA reductase B. Lipoprotein lipase C. Acetyl-CoA carboxylase D. Hormone-
sensitive lipase
Correct answer: A. HMG-CoA reductase
Explanation: HMG-CoA reductase converts HMG-CoA into mevalonate. Statins inhibit this
enzyme, reducing hepatic cholesterol synthesis and increasing LDL receptor expression.
8. Which biochemical pathway generates ribose-5-phosphate and NADPH?
A. Urea cycle B. Glycolysis C. Beta-oxidation D. Pentose phosphate pathway
Correct answer: D. Pentose phosphate pathway
Explanation: The pentose phosphate pathway generates NADPH for reductive biosynthesis
and antioxidant defense, as well as ribose-5-phosphate for nucleotide synthesis.
9. A patient has elevated lactate following severe tissue hypoxia. Why does lactate
increase?
A. Increased aerobic oxidative phosphorylation B. Increased conversion of pyruvate to
lactate to regenerate NAD+ C. Inhibition of all glycolysis D. Increased conversion of lactate
into ketones in muscle
Correct answer: B. Increased conversion of pyruvate to lactate to regenerate NAD+
Explanation: Lactate dehydrogenase converts pyruvate into lactate while regenerating
NAD+, allowing glycolysis to continue producing ATP when oxidative metabolism is
inadequate.
