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APMLE Part I study materials
APMLE Part I study resources

APMLE Part I practice questions

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Question 1Anatomy (including lower extremity anatomy, histology, and embryology)

Within the femoral triangle, the major contents are arranged in a characteristic lateral-to-medial order. Which sequence is correct?

  1. Femoral nerve, femoral artery, femoral vein, lymphatics Correct answer.
  2. Femoral nerve, femoral vein, femoral artery, lymphatics
  3. Femoral artery, femoral nerve, femoral vein, lymphatics
  4. Femoral vein, femoral artery, femoral nerve, lymphatics

Explanation

The femoral triangle contains, from lateral to medial, the femoral nerve, artery, vein, and the lymphatics within the femoral canal. This arrangement reflects the embryologic migration of the nerve into the anterior thigh and is the basis of the mnemonic NAVEL.

Question 2Anatomy (including lower extremity anatomy, histology, and embryology)

The femoral nerve arises from which spinal cord levels?

  1. L4 through S3
  2. L4 through S2
  3. L2 through L4 Correct answer.
  4. L1 through L3

Explanation

The femoral nerve is formed by the posterior divisions of the ventral rami of L2 through L4. It supplies the anterior compartment of the thigh and gives the saphenous nerve to the medial leg.

Question 3Anatomy (including lower extremity anatomy, histology, and embryology)

The anterior cruciate ligament is best described by which function?

  1. It resists varus angulation of the knee
  2. It limits flexion of the knee beyond 90 degrees
  3. It prevents posterior displacement of the tibia on the femur
  4. It prevents anterior displacement of the tibia on the femur Correct answer.

Explanation

The anterior cruciate ligament runs from the anterior intercondylar area of the tibia to the lateral femoral condyle, restraining anterior translation of the tibia relative to the femur. Its counterpart, the posterior cruciate ligament, restrains posterior translation.

Question 4Anatomy (including lower extremity anatomy, histology, and embryology)

Which statement correctly describes the origin and course of the great saphenous vein?

  1. It arises from the medial end of the dorsal venous arch and ascends in front of the medial malleolus Correct answer.
  2. It arises from the popliteal vein and ascends in the midline of the posterior thigh
  3. It arises from the deep plantar arch and ascends between the tibia and fibula
  4. It arises from the lateral end of the dorsal venous arch and ascends behind the lateral malleolus

Explanation

The great saphenous vein begins at the medial end of the dorsal venous arch, passes anterior to the medial malleolus, and ascends with the saphenous nerve to drain into the femoral vein. It is the longest vein in the body.

Question 5Anatomy (including lower extremity anatomy, histology, and embryology)

Which action is the primary function of the gluteus medius?

  1. Lateral rotation of the hip with the knee extended
  2. Adduction of the hip against resistance
  3. Abduction of the hip and stabilization of the pelvis during single-leg stance Correct answer.
  4. Extension of the hip from a flexed position

Explanation

The gluteus medius arises from the lateral ilium and inserts on the greater trochanter, acting as the chief abductor and pelvic stabilizer in stance. Weakness produces a positive Trendelenburg sign on the contralateral side.

Question 6Anatomy (including lower extremity anatomy, histology, and embryology)

A patient sustains an intracapsular fracture of the femoral neck. Disruption of which blood supply is most responsible for subsequent avascular necrosis of the femoral head?

  1. The ascending branch of the first perforating artery
  2. Retinacular branches of the medial circumflex femoral artery Correct answer.
  3. The obturator artery branch traveling in the ligamentum teres
  4. The descending branch of the lateral circumflex femoral artery

Explanation

The adult femoral head is supplied mainly by retinacular (ascending cervical) branches of the medial circumflex femoral artery, which run along the femoral neck deep to the capsule. An intracapsular fracture shears these vessels and is the classic cause of avascular necrosis.

Question 7Anatomy (including lower extremity anatomy, histology, and embryology)

In the popliteal fossa, the major neurovascular structures lie in a characteristic order from superficial (posterior) to deep (anterior). Which order is correct?

  1. Popliteal vein, tibial nerve, popliteal artery
  2. Tibial nerve, popliteal artery, popliteal vein
  3. Tibial nerve, popliteal vein, popliteal artery Correct answer.
  4. Popliteal artery, popliteal vein, tibial nerve

Explanation

From posterior to anterior the popliteal fossa contains the tibial nerve, then the popliteal vein, then the popliteal artery, which lies directly on the popliteal surface of the femur. This relationship matters clinically because a popliteal aneurysm can compress the vein and nerve.

Question 8Anatomy (including lower extremity anatomy, histology, and embryology)

Which group of structures is found within the adductor canal?

  1. Obturator nerve, femoral artery, and great saphenous vein
  2. Sciatic nerve, popliteal vein, and small saphenous vein
  3. Femoral artery, femoral vein, saphenous nerve, and nerve to vastus medialis Correct answer.
  4. Femoral nerve, femoral artery, and femoral vein

Explanation

The adductor canal (subsartorial canal) transmits the femoral artery and vein, the saphenous nerve, the nerve to vastus medialis, and lymphatics. It is bounded by vastus medialis laterally, adductor longus and magnus medially, and sartorius anteriorly.

Question 9Anatomy (including lower extremity anatomy, histology, and embryology)

Which statement correctly describes the composition of the sciatic nerve?

  1. Tibial and common fibular divisions, derived from L4 through S3 Correct answer.
  2. Tibial and common fibular divisions, derived from L2 through L4
  3. Femoral and obturator divisions, derived from L4 through S3
  4. Tibial and sural divisions, derived from S1 through S3

Explanation

The sciatic nerve is formed by the sacral plexus from L4 through S3 and consists of a tibial division and a common fibular division bound in a common sheath. The two divisions separate at or above the popliteal fossa.

Question 10Anatomy (including lower extremity anatomy, histology, and embryology)

A patient has weakness of foot eversion and sensory loss over the lateral leg and dorsum of the foot, but dorsiflexion and sensation in the first web space are preserved. Which nerve is injured?

  1. Deep fibular nerve
  2. Saphenous nerve
  3. Tibial nerve
  4. Superficial fibular nerve Correct answer.

Explanation

The superficial fibular nerve supplies fibularis longus and brevis, the everters of the foot, and provides cutaneous sensation to the lateral leg and dorsum of the foot except the first web space. Preservation of dorsiflexion and first web space sensation indicates the deep fibular nerve is intact.

Question 11Anatomy (including lower extremity anatomy, histology, and embryology)

The obturator nerve arises from which levels and supplies which compartment of the thigh?

  1. L2 through L3; anterior compartment of the thigh
  2. L4 through S2; posterior compartment of the thigh
  3. S1 through S3; posterior compartment of the leg
  4. L2 through L4; medial compartment of the thigh Correct answer.

Explanation

The obturator nerve derives from the anterior divisions of L2 through L4 and enters the medial thigh through the obturator canal to supply the adductor group, gracilis, and obturator externus. It also carries sensation from the medial thigh and the hip joint.

Question 12Anatomy (including lower extremity anatomy, histology, and embryology)

Within the sole of the foot, the tibial nerve terminates by dividing into which pair of nerves?

  1. Medial plantar and lateral plantar nerves Correct answer.
  2. Medial calcaneal and sural nerves
  3. Sural and saphenous nerves
  4. Deep fibular and superficial fibular nerves

Explanation

The tibial nerve passes behind the medial malleolus and divides in the sole into the medial and lateral plantar nerves, which supply the intrinsic muscles and skin of the sole. The medial calcaneal nerve arises slightly more proximally.

Question 13Anatomy (including lower extremity anatomy, histology, and embryology)

Which description best characterizes hyaline cartilage?

  1. Elastic fibers in a yellow matrix, found in the epiglottis and external ear
  2. Dense parallel collagen bundles with fibroblasts, found in tendons and ligaments
  3. Type I collagen organized in bundles, found in intervertebral discs and menisci
  4. Type II collagen with a glassy matrix, found on articular surfaces and in the respiratory tract Correct answer.

Explanation

Hyaline cartilage contains predominantly type II collagen in a basophilic, glassy matrix and forms articular surfaces, costal cartilage, and respiratory tract rings. Its articular surfaces lack perichondrium, limiting intrinsic repair.

Question 14Anatomy (including lower extremity anatomy, histology, and embryology)

Which description best characterizes a secondary osteon in compact bone?

  1. Concentric lamellae around a central canal with vessels Correct answer.
  2. Parallel collagen bundles with fibroblasts between them
  3. A lattice of trabeculae without a central canal
  4. A calcified cartilage core rimmed by woven bone

Explanation

A secondary osteon consists of concentric lamellae of lamellar bone surrounding a Haversian canal that carries blood vessels and nerves, with osteocytes in lacunae connected by canaliculi. Osteons are the structural units of compact bone.

Question 15Anatomy (including lower extremity anatomy, histology, and embryology)

In skeletal muscle fibers, which arrangement constitutes the triad?

  1. Three terminal cisternae surrounding a T-tubule at the H zone
  2. One T-tubule flanked by two terminal cisternae at the A-I junction Correct answer.
  3. Two T-tubules flanking one terminal cisterna at the Z line
  4. One T-tubule and one terminal cisterna at the M line

Explanation

In skeletal muscle each T-tubule invagination is flanked by two terminal cisternae of the sarcoplasmic reticulum, forming a triad located at the A-I junction. Cardiac muscle instead has diads located at the Z line.

Question 16Anatomy (including lower extremity anatomy, histology, and embryology)

Which set of adult structures is derived from the neural crest?

  1. Epidermis and its appendages
  2. Melanocytes, Schwann cells, and adrenal medulla Correct answer.
  3. Skeletal muscle and cardiac muscle
  4. Notochord and ependymal lining

Explanation

Neural crest cells migrate widely and give rise to melanocytes, Schwann cells, dorsal root and autonomic ganglia, adrenal medullary cells, and much of the craniofacial skeleton. Their broad migratory potential is why they are sometimes called the fourth germ layer.

Question 17Anatomy (including lower extremity anatomy, histology, and embryology)

Which statement about somite derivatives is correct?

  1. Myotome gives rise to dorsal root ganglia
  2. Myotome gives rise to skeletal muscle of the trunk and limbs Correct answer.
  3. Dermatome gives rise to vertebral bodies
  4. Sclerotome gives rise to the dermis of the back

Explanation

Each somite divides into sclerotome, myotome, and dermatome; the myotome forms the skeletal muscle of the trunk and limbs. The sclerotome forms vertebrae and ribs, and the dermatome forms the dermis of the back.

Question 18Anatomy (including lower extremity anatomy, histology, and embryology)

Which statement correctly describes rotation of the developing limb buds?

  1. Neither limb rotates, so dermatomes remain in the original segmental plane
  2. The upper limb rotates medially 90 degrees, placing the thumb medially
  3. The lower limb rotates medially 90 degrees, placing extensors anteriorly Correct answer.
  4. Both limbs rotate laterally 90 degrees, placing extensors anteriorly

Explanation

The lower limb bud rotates medially about 90 degrees, carrying the extensor (dorsal) compartment to the anterior thigh and leg and bringing the great toe to the medial side. The upper limb rotates laterally, placing the extensors posteriorly and the thumb laterally.

Question 19Anatomy (including lower extremity anatomy, histology, and embryology)

The anterior wall of the inguinal canal is formed principally by which structure?

  1. Aponeurosis of the external oblique Correct answer.
  2. Conjoint tendon
  3. Transversalis fascia
  4. Inguinal ligament

Explanation

The anterior wall of the inguinal canal is the aponeurosis of the external oblique, reinforced laterally by fibers of the internal oblique. The posterior wall is transversalis fascia reinforced by the conjoint tendon, and the floor is the inguinal ligament.

Question 20Anatomy (including lower extremity anatomy, histology, and embryology)

Injury to the deep fibular nerve as it winds around the fibular neck produces which deficit?

  1. Loss of knee flexion with intact dorsiflexion
  2. Loss of dorsiflexion and sensation in the first web space Correct answer.
  3. Loss of eversion with intact dorsiflexion
  4. Loss of plantar flexion with intact inversion

Explanation

The deep fibular nerve supplies the anterior compartment (tibialis anterior, extensor digitorum longus, extensor hallucis longus, fibularis tertius), producing dorsiflexion and toe extension, plus cutaneous sensation to the first web space. Injury causes footdrop with steppage gait.

Question 21Anatomy (including lower extremity anatomy, histology, and embryology)

Which statement about the deltoid (medial collateral) ligament of the ankle is correct?

  1. It resists eversion of the foot Correct answer.
  2. It consists of three separate lateral bands
  3. It resists inversion of the foot
  4. It spans from the lateral malleolus to the talus and calcaneus

Explanation

The deltoid ligament radiates from the medial malleolus to the talus, calcaneus, and navicular and is a strong restraint against eversion. Because it is so strong, forced inversion instead tears the weaker lateral ligament complex.

Question 22Anatomy (including lower extremity anatomy, histology, and embryology)

Which statement best describes the menisci of the knee?

  1. The medial meniscus is O-shaped and more mobile than the lateral
  2. The medial meniscus is C-shaped and more firmly attached to the capsule Correct answer.
  3. Both menisci are composed of hyaline cartilage
  4. The lateral meniscus is more firmly attached to the capsule and is injured more often

Explanation

The medial meniscus is C-shaped and is tethered to the joint capsule and deep fibers of the medial collateral ligament, making it less mobile and more prone to tearing. The lateral meniscus is more circular and mobile, partly because of the popliteus hiatus.

Question 23Anatomy (including lower extremity anatomy, histology, and embryology)

A variant anatomy is found in which the common fibular division of the sciatic nerve pierces the piriformis muscle. Which clinical picture is most likely?

  1. Pain and paresthesia in the gluteal region with footdrop-pattern weakness Correct answer.
  2. Loss of ankle reflex with intact sensation over the dorsum of the foot
  3. Weakness of knee flexion with sensory loss over the medial leg
  4. Weakness of hip adduction with sensory loss over the medial thigh

Explanation

When fibular fibers pass through the piriformis, contraction can compress them, producing buttock pain with tenderness and fibular-pattern deficits such as weak dorsiflexion and eversion with dorsum-of-foot sensory loss. This is the anatomic basis of piriformis syndrome.

Question 24Anatomy (including lower extremity anatomy, histology, and embryology)

Which zone of the epiphyseal growth plate shows a calcified matrix with dying chondrocytes and invasion by capillaries and osteoprogenitor cells?

  1. Proliferative zone
  2. Resting zone
  3. Hypertrophic zone
  4. Zone of calcified cartilage Correct answer.

Explanation

The zone of calcified cartilage lies adjacent to the metaphysis, where chondrocytes die, matrix calcifies, and vascular sprouts bring osteoprogenitor cells that deposit bone on the calcified spicules. This is the site where longitudinal growth is converted into new bone.

Question 25Anatomy (including lower extremity anatomy, histology, and embryology)

Which structure passes through the lesser sciatic foramen?

  1. Tendon of obturator externus
  2. Sciatic nerve
  3. Superior gluteal nerve
  4. Tendon of obturator internus Correct answer.

Explanation

The lesser sciatic foramen transmits the tendon of obturator internus along with the nerve to obturator internus, the pudendal nerve, and the internal pudendal vessels. It lies between the sacrotuberous and sacrospinous ligaments below the pelvic floor.

Question 26Biochemistry

Which amino acid has an imidazole side chain that buffers most effectively near physiologic pH?

  1. Cysteine
  2. Glutamate
  3. Histidine Correct answer.
  4. Lysine

Explanation

The imidazole group of histidine has a pKa of about 6.0, close enough to physiologic pH that it exists in both protonated and unprotonated forms and can absorb or release protons. Side chains whose pKa values lie far from 7.4 are essentially fully protonated or fully deprotonated and buffer poorly.

Question 27Biochemistry

Which enzyme catalyzes the rate-limiting step of glycolysis?

  1. Hexokinase
  2. Pyruvate kinase
  3. Phosphofructokinase-1 Correct answer.
  4. Glyceraldehyde-3-phosphate dehydrogenase

Explanation

Phosphofructokinase-1 converts fructose-6-phosphate to fructose-1,6-bisphosphate and sets the pace of glycolytic flux; it is activated by AMP and fructose-2,6-bisphosphate and inhibited by ATP and citrate. The other enzymes of the pathway carry the flux that PFK-1 determines.

Question 28Biochemistry

A newborn has prolonged prothrombin time and bleeding attributed to deficiency of a fat-soluble vitamin that crosses the placenta poorly. Which vitamin is deficient?

  1. Vitamin A
  2. Vitamin C
  3. Vitamin K Correct answer.
  4. Vitamin E

Explanation

Vitamin K is the required cofactor for the gamma-carboxylase that modifies glutamate residues in factors II, VII, IX, and X, allowing them to bind calcium and phospholipid. Without it, these factors are secreted but functionally inactive, prolonging the prothrombin time.

Question 29Biochemistry

Which enzyme separates the two strands of the DNA double helix ahead of the replication fork?

  1. Topoisomerase
  2. Helicase Correct answer.
  3. Primase
  4. DNA ligase

Explanation

Helicase uses ATP to disrupt the hydrogen bonds between complementary bases, generating the single-stranded templates that polymerases require. The other enzymes act on primers, supercoils, or nicks rather than on base pairing itself.

Question 30Biochemistry

Which molecular change in the beta-globin chain produces sickle cell hemoglobin?

  1. Lysine is replaced by glutamate at position 6
  2. Valine is replaced by glutamate at position 6
  3. Glutamate at position 6 is deleted
  4. Glutamate is replaced by valine at position 6 Correct answer.

Explanation

A single base transversion in codon 6 changes glutamate to valine, creating a hydrophobic contact point on the surface of deoxyhemoglobin S. These contacts allow polymerization of deoxygenated hemoglobin into rigid fibers that deform the red cell.

Question 31Biochemistry

How many net molecules of ATP are generated per molecule of glucose converted to lactate by anaerobic glycolysis?

  1. 4
  2. 1
  3. 3
  4. 2 Correct answer.

Explanation

Four ATP molecules are produced by substrate-level phosphorylation, but two are consumed, one by hexokinase and one by phosphofructokinase-1. The net yield is therefore two ATP, which is why anaerobic glycolysis is an inefficient but rapid source of energy.

Question 32Biochemistry

Which molecule is the common two-carbon unit that enters the citric acid cycle from the catabolism of carbohydrates, fats, and amino acids?

  1. Acetyl-CoA Correct answer.
  2. Succinyl-CoA
  3. alpha-Ketoglutarate
  4. Pyruvate

Explanation

Acetyl-CoA condenses with oxaloacetate in the citrate synthase reaction, and this is the point at which carbon from all three macronutrient classes converges on the cycle. Pyruvate, succinyl-CoA, and alpha-ketoglutarate are either precursors or intermediates that cannot serve this converging role.

Question 33Biochemistry

Which cofactor is required by the aminotransferases ALT and AST?

  1. Tetrahydrofolate
  2. Thiamine pyrophosphate
  3. Pyridoxal phosphate Correct answer.
  4. Biotin

Explanation

Pyridoxal phosphate, the active form of vitamin B6, forms a Schiff base with the amino acid substrate and temporarily carries the amino group before transferring it to an alpha-keto acid. The other cofactors serve decarboxylation, carboxylation, or one-carbon transfer reactions instead.

Question 34Biochemistry

For an enzyme obeying Michaelis-Menten kinetics, what is the reaction velocity when the substrate concentration equals Km?

  1. Twice Vmax
  2. One-quarter of Vmax
  3. Vmax
  4. One-half of Vmax Correct answer.

Explanation

Km is defined operationally as the substrate concentration that produces half-maximal velocity, which follows directly from substituting [S] = Km into the Michaelis-Menten equation. Velocity approaches but never exceeds Vmax, so values above Vmax are not attainable.

Question 35Biochemistry

A competitive inhibitor is added to an enzyme-catalyzed reaction. How are the apparent kinetic constants affected?

  1. Km and Vmax both increase
  2. Km increases and Vmax is unchanged Correct answer.
  3. Vmax decreases and Km is unchanged
  4. Km and Vmax both decrease

Explanation

A competitive inhibitor occupies the active site reversibly, so its effect can be overcome by raising substrate concentration; Vmax is therefore unchanged while more substrate is needed to reach half-saturation, raising the apparent Km. This pattern produces intersecting lines at the y-axis on a Lineweaver-Burk plot.

Question 36Biochemistry

Which enzyme catalyzes the committed, irreversible step of the citric acid cycle?

  1. Fumarase
  2. Citrate synthase Correct answer.
  3. Malate dehydrogenase
  4. Aconitase

Explanation

Citrate synthase condenses oxaloacetate with acetyl-CoA to form citrate, and this is the step that commits carbon to the cycle. It is strongly regulated, being inhibited by ATP, NADH, and succinyl-CoA, whereas the other enzymes listed catalyze near-equilibrium reversible reactions.

Question 37Biochemistry

An infant with pyruvate dehydrogenase deficiency has lactic acidosis and neurologic impairment. Which metabolic pattern is expected?

  1. Accumulation of lactate and pyruvate with decreased acetyl-CoA Correct answer.
  2. Increased acetyl-CoA with low pyruvate
  3. Increased flux through the citric acid cycle
  4. Low lactate with increased ketone body use

Explanation

Pyruvate dehydrogenase is the irreversible link between glycolysis and the citric acid cycle, converting pyruvate to acetyl-CoA. When it is deficient, pyruvate cannot enter the cycle, so it is reduced to lactate and cycle flux falls, which also explains the neurologic dependence on lactate and ketones.

Question 38Biochemistry

A patient with glucose-6-phosphate dehydrogenase deficiency develops hemolytic anemia after taking an oxidant drug. Which mechanism best explains the hemolysis?

  1. Reduced NADPH production impairs regeneration of reduced glutathione Correct answer.
  2. Excess NADPH promotes peroxidation of membrane lipids
  3. Impaired glycolysis lowers 2,3-bisphosphoglycerate
  4. Reduced ATP production impairs the sodium-potassium pump

Explanation

Glucose-6-phosphate dehydrogenase is the rate-limiting enzyme of the pentose phosphate pathway and the main source of NADPH in red cells. NADPH keeps glutathione reduced, and reduced glutathione detoxifies peroxides; without it, oxidant stress destroys the membrane and hemoglobin.

Question 39Biochemistry

During active hepatic fatty acid synthesis in the fed state, mitochondrial fatty acid oxidation is limited mainly by which change?

  1. Increased malonyl-CoA inhibits carnitine palmitoyltransferase I Correct answer.
  2. Increased carnitine stimulates mitochondrial fatty acid uptake
  3. Increased palmitoyl-CoA activates carnitine palmitoyltransferase I
  4. Decreased insulin activates hormone-sensitive lipase

Explanation

Malonyl-CoA is the first committed product of fatty acid synthesis and is a potent inhibitor of carnitine palmitoyltransferase I, the gatekeeper of mitochondrial fatty acid entry. This reciprocal control prevents a futile cycle in which newly synthesized fatty acids are simultaneously oxidized.

Question 40Biochemistry

Which enzyme catalyzes the rate-limiting step of de novo fatty acid synthesis?

  1. Fatty acid synthase
  2. HMG-CoA reductase
  3. Acetyl-CoA carboxylase Correct answer.
  4. Carnitine palmitoyltransferase I

Explanation

Acetyl-CoA carboxylase converts acetyl-CoA to malonyl-CoA, the committed and regulated step of fatty acid synthesis; it is activated by citrate and inhibited by palmitoyl-CoA. Fatty acid synthase performs the subsequent condensation and reduction cycle but does not set the pathway's rate.

Question 41Biochemistry

A neonate has hyperammonemia, respiratory alkalosis, elevated urine orotic acid, and low plasma citrulline. Which enzyme is most likely deficient?

  1. Argininosuccinate lyase
  2. Carbamoyl phosphate synthetase I
  3. Arginase
  4. Ornithine transcarbamylase Correct answer.

Explanation

Ornithine transcarbamylase transfers carbamoyl phosphate to ornithine; when it is deficient, carbamoyl phosphate accumulates in mitochondria, spills into the cytosol, and is shunted into pyrimidine synthesis, producing orotic acid. This X-linked disorder is the most common inherited urea cycle defect.

Question 42Biochemistry

A boy with intellectual disability, spasticity, and self-injurious biting is found to have hyperuricemia. Which enzyme is deficient?

  1. HGPRT Correct answer.
  2. PRPP synthetase
  3. Xanthine oxidase
  4. Adenosine deaminase

Explanation

Hypoxanthine-guanine phosphoribosyltransferase salvages hypoxanthine and guanine by transferring them to phosphoribosyl pyrophosphate; when it is deficient, these purines are degraded to uric acid instead of recycled. The resulting Lesch-Nyhan syndrome combines hyperuricemia with neurologic and behavioral abnormalities.

Question 43Biochemistry

Which enzyme catalyzes the rate-limiting step of heme biosynthesis in hepatocytes?

  1. Porphobilinogen deaminase
  2. ALA dehydratase
  3. ALA synthase Correct answer.
  4. Ferrochelatase

Explanation

Mitochondrial aminolevulinate synthase condenses glycine with succinyl-CoA to form delta-aminolevulinic acid, and it is inhibited by heme and by glucose loading. The remaining enzymes of the pathway act downstream and are not rate limiting.

Question 44Biochemistry

Vitamin C is required for which reaction during collagen synthesis?

  1. Cleavage of the signal peptide during secretion
  2. Gamma-carboxylation of glutamate residues
  3. Formation of disulfide bonds between cysteine residues
  4. Hydroxylation of proline and lysine residues Correct answer.

Explanation

Ascorbate is the reducing cofactor for prolyl and lysyl hydroxylase, which add hydroxyl groups that stabilize the collagen triple helix through interchain hydrogen bonds. Its absence impairs cross-linking and produces the fragile connective tissue of scurvy.

Question 45Biochemistry

Which action of insulin directly increases glucose uptake in skeletal muscle and adipose tissue?

  1. Translocation of GLUT4 to the plasma membrane Correct answer.
  2. Stimulation of hepatic glycogenolysis
  3. Activation of hormone-sensitive lipase
  4. Increased expression of gluconeogenic enzymes

Explanation

Insulin signaling through the PI3K-Akt pathway triggers exocytosis of GLUT4-containing vesicles, raising the number of glucose transporters at the cell surface. This transport step is the rate-limiting determinant of glucose entry in muscle and fat.

Question 46Biochemistry

An uncompetitive inhibitor binds only to the enzyme-substrate complex. How does it change the apparent kinetic constants?

  1. Km decreases and Vmax increases
  2. Km and Vmax both decrease Correct answer.
  3. Km increases, Vmax unchanged
  4. Km is unchanged and Vmax decreases

Explanation

Uncompetitive inhibition removes enzyme-substrate complex from the reaction, lowering Vmax, and preferential binding to that complex lowers the apparent Km. On a Lineweaver-Burk plot this gives parallel lines, which distinguishes it from competitive and noncompetitive patterns.

Question 47Biochemistry

A 5-day-old infant has poor feeding, lethargy, and urine with a maple syrup odor. Which enzyme is deficient?

  1. Methylmalonyl-CoA mutase
  2. Branched-chain alpha-ketoacid dehydrogenase Correct answer.
  3. Isovaleryl-CoA dehydrogenase
  4. Pyruvate dehydrogenase complex

Explanation

Branched-chain alpha-ketoacid dehydrogenase decarboxylates the keto acids of leucine, isoleucine, and valine; its deficiency causes these metabolites to accumulate and gives the characteristic sweet odor. The disorder is autosomal recessive and presents in the first week of life with encephalopathy.

Question 48Biochemistry

Methotrexate produces its anticancer effect primarily by inhibiting which enzyme?

  1. Ribonucleotide reductase
  2. Dihydrofolate reductase Correct answer.
  3. Topoisomerase II
  4. Thymidylate synthase

Explanation

Methotrexate is a folate analog that binds dihydrofolate reductase with high affinity, depleting tetrahydrofolate and thereby blocking thymidylate and purine synthesis. Cells with high turnover are most vulnerable, which underlies both its antineoplastic action and its toxicity to gut and marrow.

Question 49Biochemistry

Patients with xeroderma pigmentosum have extreme photosensitivity and multiple skin cancers because of a defect in which process?

  1. Homologous recombination repair
  2. Nucleotide excision repair Correct answer.
  3. Mismatch repair
  4. Base excision repair

Explanation

Ultraviolet light creates bulky pyrimidine dimers that are removed by nucleotide excision repair, a multistep process that excises an oligonucleotide patch containing the lesion. When this pathway fails, unrepaired dimers cause mutations that lead to actinic damage and skin malignancy.

Question 50Biochemistry

A child with severe fasting hypoglycemia, lactic acidosis, hyperuricemia, and hepatomegaly is found to have a glycogen storage disorder. Which enzyme is deficient?

  1. Muscle glycogen phosphorylase
  2. Glycogen debranching enzyme
  3. Lysosomal acid alpha-glucosidase
  4. Glucose-6-phosphatase Correct answer.

Explanation

Glucose-6-phosphatase is the terminal step of both glycogenolysis and gluconeogenesis in liver and kidney; without it, glucose-6-phosphate cannot be released as free glucose. The trapped intermediate is diverted into glycogen, lactate, and purine degradation, explaining hepatomegaly, lactic acidosis, and hyperuricemia.