📝 Step 5 — KMU Past Papers & Exam Learning

This section contains KMU-style past paper questions designed to strengthen conceptual understanding. Focus on understanding explanations rather than memorizing answers.

🎯 How to Study KMU Past Papers

• Read the question carefully.
• Think about the answer before looking.
• Read the explanation slowly.
• Understand the reasoning behind the correct answer.
• Revise difficult questions again.

MCQ 1

Question:
A child with untreated phenylalanine metabolism defect develops intellectual disability and hypopigmentation. The reduced pigmentation is mainly due to decreased formation of:

Options:
Serotonin
Melanin
Creatinine
Glutathione
Urea

Correct Answer:
Melanin

Explanation:
Phenylalanine is normally converted to tyrosine, which is needed for melanin synthesis. Reduced tyrosine availability contributes to hypopigmentation in PKU.

MCQ 2

Question:
A newborn screening test detects elevated phenylalanine. Which dietary approach is most important to prevent neurological damage?

Options:
High protein diet
Low tyrosine diet
Low phenylalanine diet
High methionine diet
Low cysteine diet

Correct Answer:
Low phenylalanine diet

Explanation:
In PKU, phenylalanine accumulates and damages the developing brain. Early dietary restriction prevents severe neurological complications.

MCQ 3

Question:
A patient has blackening of urine after standing and ochronotic pigmentation of cartilage. The accumulated metabolite is:

Options:
Phenylpyruvate
Homogentisic acid
Homocysteine
Cystine
Kynurenine

Correct Answer:
Homogentisic acid

Explanation:
Alkaptonuria results from homogentisate oxidase deficiency, causing accumulation of homogentisic acid.

MCQ 4

Question:
A defect in conversion of tyrosine to DOPA will most directly impair synthesis of:

Options:
Melanin
Urea
Glutathione
Creatine
Cystathionine

Correct Answer:
Melanin

Explanation:
Tyrosinase converts tyrosine to DOPA in melanin synthesis. Deficiency causes albinism.

MCQ 5

Question:
Tryptophan metabolism is clinically important because it contributes to synthesis of:

Options:
Thyroxine and melanin
Serotonin and niacin
Glutathione and taurine
Cystine and creatine
Dopamine and epinephrine

Correct Answer:
Serotonin and niacin

Explanation:
Tryptophan forms serotonin, melatonin, and niacin. Reduced tryptophan availability may produce pellagra-like features.

MCQ 6

Question:
A patient with Hartnup disease develops dermatitis and neurological symptoms. The biochemical basis is reduced availability of tryptophan for synthesis of:

Options:
Vitamin B₃
Vitamin B₆
Vitamin B₁₂
Vitamin C
Vitamin K

Correct Answer:
Vitamin B₃

Explanation:
Tryptophan can form niacin. Defective tryptophan transport may reduce niacin synthesis and cause pellagra-like symptoms.

MCQ 7

Question:
Which amino acid metabolism provides the major methyl donor for DNA and neurotransmitter methylation reactions?

Options:
Cysteine
Tyrosine
Methionine
Tryptophan
Phenylalanine

Correct Answer:
Methionine

Explanation:
Methionine forms S-adenosyl methionine, the major methyl donor in many methylation reactions.

MCQ 8

Question:
A child has lens dislocation, thrombotic tendency, and marfanoid habitus. The most likely accumulated amino acid derivative is:

Options:
Homocysteine
Homogentisate
Phenylpyruvate
Cystine
Tyrosine

Correct Answer:
Homocysteine

Explanation:
Homocystinuria causes accumulation of homocysteine, leading to thrombosis, lens dislocation, and skeletal abnormalities.

MCQ 9

Question:
Vitamin B₆ therapy may help some patients with homocystinuria because it supports the pathway from homocysteine toward:

Options:
Phenylalanine
Cysteine
Tryptophan
Tyrosine
Melanin

Correct Answer:
Cysteine

Explanation:
Vitamin B₆ is required for transsulfuration of homocysteine toward cysteine.

MCQ 10

Question:
Cysteine is important in maintaining protein structure mainly because it forms:

Options:
Peptide bonds
Glycosidic bonds
Disulfide bonds
Hydrogen bonds
Ester bonds

Correct Answer:
Disulfide bonds

Explanation:
Cysteine residues form disulfide bonds, which stabilize tertiary and quaternary protein structure.

MCQ 11

Question:
A patient has recurrent renal stones composed of poorly soluble cystine. The primary defect is in:

Options:
Hepatic hydroxylation
Renal tubular transport
Intestinal peptide digestion
Mitochondrial oxidation
Urea cycle regulation

Correct Answer:
Renal tubular transport

Explanation:
Cystinuria is due to defective renal tubular reabsorption of cystine and related amino acids, causing cystine stones.

MCQ 12

Question:
In the urea cycle, ammonia first combines with carbon dioxide in mitochondria to form:

Options:
Argininosuccinate
Citrulline
Carbamoyl phosphate
Ornithine
Arginine

Correct Answer:
Carbamoyl phosphate

Explanation:
Carbamoyl phosphate synthetase I forms carbamoyl phosphate from ammonia and carbon dioxide in the mitochondrial matrix.

MCQ 13

Question:
Which molecule activates carbamoyl phosphate synthetase I in the urea cycle?

Options:
N-acetylglutamate
S-adenosyl methionine
Tetrahydrobiopterin
Pyridoxal phosphate
Biotin

Correct Answer:
N-acetylglutamate

Explanation:
N-acetylglutamate is the essential activator of carbamoyl phosphate synthetase I.

MCQ 14

Question:
A patient with urea cycle enzyme deficiency develops vomiting, confusion, and cerebral edema. These features are mainly due to accumulation of:

Options:
Lactate
Ammonia
Ketone bodies
Bilirubin
Oxalate

Correct Answer:
Ammonia

Explanation:
Urea cycle defects impair ammonia detoxification, producing hyperammonemia and neurological dysfunction.

MCQ 15

Question:
Which urea cycle intermediate is formed from ornithine and carbamoyl phosphate?

Options:
Arginine
Citrulline
Fumarate
Aspartate
Urea

Correct Answer:
Citrulline

Explanation:
Ornithine transcarbamylase combines ornithine with carbamoyl phosphate to form citrulline.

MCQ 16

Question:
Which amino acid donates the second nitrogen atom during urea synthesis?

Options:
Glutamate
Aspartate
Alanine
Glycine
Serine

Correct Answer:
Aspartate

Explanation:
One nitrogen of urea comes from free ammonia, and the second nitrogen comes from aspartate.

MCQ 17

Question:
Argininosuccinate cleavage in the urea cycle produces arginine and:

Options:
Fumarate
Malate
Citrate
Oxaloacetate
Succinate

Correct Answer:
Fumarate

Explanation:
Argininosuccinate lyase produces arginine and fumarate, linking the urea cycle with the TCA cycle.

MCQ 18

Question:
A defect in ornithine transcarbamylase most directly prevents formation of:

Options:
Citrulline
Arginine
Urea
Fumarate
Aspartate

Correct Answer:
Citrulline

Explanation:
Ornithine transcarbamylase catalyzes formation of citrulline from ornithine and carbamoyl phosphate.

MCQ 19

Question:
The liver is central in amino acid metabolism because it:

Options:
Stores free amino acids permanently
Converts toxic ammonia into urea
Synthesizes all essential amino acids
Prevents protein turnover in muscle
Eliminates nitrogen through bile

Correct Answer:
Converts toxic ammonia into urea

Explanation:
The liver detoxifies ammonia by converting it into urea for renal excretion.

MCQ 20

Question:
A combined understanding of amino acid catabolism and the urea cycle explains why severe liver failure commonly causes:

Options:
Hyperammonemic encephalopathy
Hypocalcemic tetany
Iron overload anemia
Renal cystine stones
Melanin excess

Correct Answer:
Hyperammonemic encephalopathy

Explanation:
Liver failure impairs urea formation, causing ammonia accumulation and neurological dysfunction.

📌 Important Exam Strategy

KMU examinations often test integrated understanding rather than isolated facts. Focus on linking anatomy, embryology, histology, and clinical concepts when reviewing questions.

✅ Revision Tip

If you can explain the reason behind the correct answer without looking at notes, your concept is strong.