Concours de fin de PACES — Médecine Humaine (Université d'Antananarivo)

Which of the following statements correctly describes the structural characteristics of the peptide bond in proteins?

• AIt is a freely rotating single bond between the alpha-carbon and the carbonyl carbon.
• BIt is formed by a hydrolysis reaction releasing a molecule of carbon dioxide.
• CIt possesses partial double-bond character due to resonance, resulting in a planar geomet…
• DIt predominantly adopts a cis configuration in polypeptide backbones due to favorable ste…

Under physiological conditions, what are the canonical geometric parameters of the B-form DNA double helix described by Watson and Crick?

• AA left-handed helix with 12 base pairs per turn and a pitch of 4.5 nm.
• BA right-handed helix with approximately 10.5 base pairs per helical turn and a pitch of 3…
• CA right-handed helix with 11 base pairs per turn and base pairs tilted at 20° to the heli…
• DA triple-stranded helix stabilized by non-canonical Hoogsteen base pairing throughout the…

In enzyme kinetics following the Michaelis-Menten model, how does the addition of a reversible competitive inhibitor alter the apparent Michaelis constant (Km) and the maximum velocity (Vmax)?

• AKm increases (apparent affinity decreases) while Vmax remains unchanged.
• BBoth Km and Vmax decrease proportionally, leaving the ratio Vmax/Km constant.
• CVmax decreases while Km remains completely unchanged.
• DKm decreases (apparent affinity increases) while Vmax increases.

Which glycolytic enzyme catalyzes the primary committed and rate-limiting step of glycolysis, and what is its potent physiological allosteric activator?

• AHexokinase; activated by glucose-6-phosphate.
• BPyruvate kinase; activated by high levels of intracellular ATP and acetyl-CoA.
• CGlyceraldehyde 3-phosphate dehydrogenase; activated by NADH and lactate.
• DPhosphofructokinase-1 (PFK-1); activated by fructose-2,6-bisphosphate and AMP.

During the tricarboxylic acid (Krebs) cycle, which reaction catalyzes the first oxidative decarboxylation, generating the first molecule of NADH and releasing CO2?

• AConversion of citrate to isocitrate by aconitase.
• BCondensation of oxaloacetate and acetyl-CoA by citrate synthase.
• CConversion of isocitrate to alpha-ketoglutarate by isocitrate dehydrogenase.
• DConversion of succinyl-CoA to succinate by succinyl-CoA synthetase.

In the mitochondrial electron transport chain, which enzyme complex catalyzes the terminal transfer of four electrons to molecular oxygen to produce two molecules of water?

• AComplex IV (Cytochrome c oxidase).
• BComplex I (NADH:ubiquinone oxidoreductase).
• CComplex II (Succinate dehydrogenase).
• DComplex III (Cytochrome bc1 complex / CoQH2-cytochrome c reductase).

What is the biochemical mechanism by which uncoupling protein 1 (UCP1 / thermogenin) dissipates energy in brown adipose tissue mitochondria?

• AIt directly binds and inhibits the catalytic beta subunits of the F1 ATPase domain.
• BIt provides a proton leak pathway across the inner mitochondrial membrane, bypassing ATP…
• CIt irreversibly blocks Complex III, halting the Q-cycle and forcing electrons to reduce m…
• DIt hydrolyzes cytosolic ATP into AMP and inorganic pyrophosphate to generate exothermic t…

In hepatic gluconeogenesis, which two sequential enzymes bypass the irreversible glycolytic reaction catalyzed by pyruvate kinase, and what obligate cofactor is required by the first enzyme?

• APyruvate dehydrogenase and malate dehydrogenase; requiring thiamine pyrophosphate (TPP).
• BPhosphofructokinase-2 and fructose-1,6-bisphosphatase; requiring pyridoxal phosphate (PLP…
• CPyruvate carboxylase (requiring biotin) and phosphoenolpyruvate carboxykinase (PEPCK).
• DGlucose-6-phosphatase and lactate dehydrogenase; requiring tetrahydrofolate (THF).

What is the rate-limiting transport mechanism that transfers long-chain fatty acyl-CoA molecules across the inner mitochondrial membrane for beta-oxidation?

• APassive diffusion through large porin channels in the inner mitochondrial membrane.
• BThe citrate-malate shuttle using ATP-citrate lyase.
• CThe glycerol-3-phosphate shuttle utilizing cytosolic and mitochondrial FAD.
• DThe carnitine shuttle, initiated by carnitine palmitoyltransferase I (CPT-I) on the outer…

How does cholesterol modulate the physical state and fluidity of eukaryotic plasma membranes across varying physiological temperatures?

• AIt acts as a bidirectional fluidity buffer, restraining phospholipid mobility at high tem…
• BIt always increases membrane fluidity regardless of temperature by cleaving saturated fat…
• CIt completely eliminates the phase transition temperature (Tm) by converting the lipid bi…
• DIt displaces integral membrane proteins into the extracellular space when temperature dro…

What is the exact ion stoichiometry and electrogenic nature of the primary active transport cycle mediated by the Na+/K+-ATPase pump?

• AIt exports 2 Na+ ions and imports 3 K+ ions per hydrolyzed ATP, generating a net positive…
• BIt exports 3 Na+ ions and imports 2 K+ ions per hydrolyzed ATP, generating a net outward…
• CIt transports 1 Na+ ion and 1 K+ ion in electroneutral antiport per hydrolyzed ATP.
• DIt cotransports 3 Na+ and 2 K+ in the same direction into the cytosol without net charge…

During co-translational translocation into the rough endoplasmic reticulum (RER), what component recognizes the N-terminal hydrophobic signal peptide and temporarily arrests translation?

• AThe Sec61 translocon pore complex.
• BThe signal peptidase enzyme complex.
• CThe signal recognition particle (SRP).
• DThe BiP / GRP78 chaperone protein.

Which biochemical modification occurring in the cis-Golgi network serves as the specific molecular sorting tag that targets acid hydrolase precursors to lysosomes?

• AAddition of a core polyadenylation tail.
• BRemoval of all terminal galactose residues.
• CO-linked glycosylation with sulfated glucuronic acid.
• DPhosphorylation of terminal mannose residues to produce mannose-6-phosphate (M6P).

How is the acidic internal environment (pH ~4.5 to 5.0) of lysosomes maintained to ensure optimal activity of acid hydrolases?

• ABy primary active transport of protons into the lysosomal lumen via a vacuolar-type H+-AT…
• BBy passive inward diffusion of carbonic acid from the cytoplasm down a concentration grad…
• CBy continuous enzymatic conversion of lactic acid to hydrochloric acid by cytoplasmic LDH.
• DBy sodium-hydrogen antiporters that utilize extracellular ATP to pump protons out of the…

Which metabolic function is uniquely performed by peroxisomes, and which enzyme breaks down the toxic hydrogen peroxide byproduct produced during these reactions?

• AComplete oxidation of acetyl-CoA to CO2; regulated by cytochrome c oxidase.
• BInitial alpha- and beta-oxidation of very-long-chain fatty acids (VLCFAs >= C22); broken…
• CGlycogen synthesis from glucose-1-phosphate; regulated by glycogen synthase.
• DOxidative phosphorylation generating ATP via proton-motive force; mediated by thermogenin.

Which statement correctly describes the structural assembly, polarity, and motor protein trafficking of cellular microtubules?

• AMicrotubules are unpolarized polymers made of actin monomers, with kinesin transporting c…
• BMicrotubules are composed of 7 protofilaments of myosin heads that hydrolyze ATP to drive…
• CMicrotubules are polarized hollow cylinders of 13 protofilaments of alpha/beta-tubulin he…
• DMicrotubules have a fast-growing minus end anchored in the plasma membrane and a stable p…

Which cyclin-CDK complex represents the maturation-promoting factor (MPF) responsible for driving the eukaryotic cell through the G2/M transition into mitosis?

• ACyclin B / CDK1.
• BCyclin D / CDK4.
• CCyclin E / CDK2.
• DCyclin A / CDK2.

In the intrinsic (mitochondrial) pathway of apoptosis, what biochemical event triggers the assembly of the heptameric apoptosome complex?

• ADirect binding of Fas ligand (FasL) to the Fas death receptor on the plasma membrane.
• BPhosphorylation of Bcl-2 by MAP kinase to block outer mitochondrial membrane permeabiliza…
• CNuclear translocation of beta-catenin following degradation of the destruction complex.
• DCytochrome c release into the cytosol following Bax/Bak oligomerization, which binds Apaf…

Which cell junction provides mechanical cohesion between adjacent epithelial cells by anchoring the transmembrane cadherins (desmoglein and desmocollin) to intermediate filaments?

• ATight junctions (zonula occludens).
• BDesmosomes (macula adherens).
• CGap junctions (nexus).
• DHemidesmosomes.

Which type of epithelium consists of a single layer of flattened, plate-like cells resting on a basal lamina, specifically adapted for rapid passive diffusion or filtration?

• AStratified squamous non-keratinized epithelium.
• BSimple columnar ciliated epithelium.
• CSimple squamous epithelium.
• DTransitional epithelium (urothelium).

What histological feature characterizes pseudostratified columnar epithelium, such as that lining the human trachea and bronchi?

• AAll cells rest directly on the basement membrane, but not all cells reach the apical surf…
• BCells are arranged in three to five true independent layers with no basal lamina.
• CSurface cells are dead, flattened squames packed with dense eleidin and keratin sheets.
• DThe apical cell layer consists of cuboidal cells specialized in storing glycogen without…

How does holocrine secretion differ from merocrine (eccrine) and apocrine secretion mechanisms in human exocrine glands?

• AThe secretory product is released by exocytosis of membrane-bound vesicles without any lo…
• BThe apical portion of the secretory cell pinches off and becomes part of the secretion wh…
• CThe secretion occurs exclusively through non-vesicular carrier proteins embedded in the b…
• DThe entire secretory cell disintegrates and discharges its accumulated contents and cell…

Which collagen type is the predominant structural component of hyaline cartilage, resisting compressive biomechanical stresses?

• AType I collagen.
• BType II collagen.
• CType III collagen.
• DType IV collagen.

Which physiological and histological characteristic distinguishes normal adult hyaline cartilage from vascularized connective tissues?

• AIt is completely avascular, aneural, and lacks lymphatic drainage, relying entirely on pa…
• BIt contains a dense network of fenestrated capillaries coursing through Haversian-like ca…
• CChondrocytes divide rapidly every 24 hours in adults to repair focal mechanical defects.
• DIt is innervated by myelinated sensory nerve fibers that terminate in specialized chondro…

Which cell type in bone tissue is a large, multinucleated giant cell derived from the monocyte-macrophage hematopoietic lineage, responsible for bone resorption in Howship lacunae?

• AOsteoblast.
• BOsteocyte.
• COsteoclast.
• DOsteoprogenitor cell.

A clinician prepares an intravenous solution containing 0.15 mol/L of sodium chloride (NaCl) and 0.05 mol/L of calcium chloride (CaCl2). Assuming complete dissociation of both salts into ideal ions, what is the total theoretical osmolarity of this solution?

• A0.20 Osm/L (200 mOsm/L).
• B0.45 Osm/L (450 mOsm/L).
• C0.35 Osm/L (350 mOsm/L).
• D0.60 Osm/L (600 mOsm/L).

According to the Van 't Hoff law of osmotic pressure and Starling's equation across capillary membranes, what is the principal determinant of plasma colloid osmotic (oncotic) pressure?

• ADissolved sodium and chloride electrolytes, which create ~280 mmHg of oncotic pressure.
• BBlood urea nitrogen and circulating free glucose.
• CHigh-density lipoproteins (HDL) that actively pump water out of the interstitial space.
• DCirculating plasma proteins (predominantly albumin), which generate ~25 to 28 mmHg of onc…

An arterial blood gas sample reveals a plasma bicarbonate concentration [HCO3-] of 24 mmol/L and a dissolved carbon dioxide concentration [CO2] of 1.2 mmol/L. Using the Henderson-Hasselbalch equation (pH = pKa + log([HCO3-]/[CO2])) with a pKa of 6.10, what is the calculated arterial pH?

• A7.40.
• B7.10.
• C7.60.
• D7.25.

Under Poiseuille's law of laminar hemodynamic flow (R = 8*eta*L / (pi * r^4)), how does a 50% reduction in the internal radius (r) of an arteriole affect its vascular resistance to blood flow?

• AResistance doubles (increases by a factor of 2).
• BResistance increases by a factor of 16 (increases to 1600% of baseline).
• CResistance increases by a factor of 4.
• DResistance increases by a factor of 8.

The radioactive isotope Iodine-131 (131-I), used in thyroid scintigraphy and ablation, has a physical half-life (T_1/2) of 8 days. What fraction of the original radioactivity remains in a sealed source after exactly 24 days?

• A50% (1/2).
• B25% (1/4).
• C6.25% (1/16).
• D12.5% (1/8).

According to the Beer-Lambert attenuation law for monochromatic X-rays (I = I0 * e^(-mu * x)), what mathematical relationship links the linear attenuation coefficient (mu) to the half-value layer (HVL)?

• AHVL = ln(2) / mu ≈ 0.693 / mu.
• BHVL = mu / ln(2) ≈ mu / 0.693.
• CHVL = 1 / (mu^2).
• DHVL = e^(-mu) / 2.

According to Planck's quantum theory (E = h * f = h * c / lambda), what happens to the energy carried by an electromagnetic photon when its wavelength (lambda) is halved?

• AThe photon energy is halved.
• BThe photon energy is doubled.
• CThe photon energy increases by a factor of 4.
• DThe photon energy remains constant because photon energy is independent of wavelength.

In a medical diagnostic X-ray tube with a tungsten anode, how is the continuous bremsstrahlung (braking radiation) spectrum produced?

• ABy radioactive alpha decay occurring within the tungsten target nucleus.
• BBy the ejection of K-shell electrons followed by L-to-K orbital transitions that release…
• CBy rapid deceleration and deflection of incident high-speed projectile electrons within t…
• DBy nuclear fission of tungsten isotopes releasing gamma-ray photons.

In radiation protection and dosimetry, which unit of measurement quantifies the equivalent dose (H), taking into account the biological damaging effectiveness of different radiation types (radiation weighting factor w_R)?

• AThe Gray (Gy), defined as 1 Joule of absorbed energy per kilogram.
• BThe Becquerel (Bq), defined as 1 nuclear disintegration per second.
• CThe Roentgen (R), defined as the electric charge produced by ionization per kilogram of a…
• DThe Sievert (Sv), defined as the absorbed dose in Grays multiplied by the radiation weigh…

Under the Snell-Descartes law of refraction (n1 * sin(theta1) = n2 * sin(theta2)), what two conditions are required for total internal reflection to occur at an optical interface?

• AThe light must travel from a medium with a higher refractive index to one with a lower re…
• BThe light must travel from a lower index medium to a higher index medium (n1 < n2) at nor…
• CBoth media must have identical refractive indices (n1 = n2) and equal absorption coeffici…
• DThe incident beam must be circularly polarized with a wavelength strictly under 200 nm.

A new diagnostic serological test for an infectious disease is evaluated in a cohort of 100 confirmed infected patients. The test correctly returns a positive result in 90 of these patients and is falsely negative in 10. What is the clinical sensitivity of this diagnostic test?

• A10%.
• B90%.
• C81.8%.
• D95%.

In a validation study of a screening test, 200 confirmed healthy (non-diseased) control individuals are tested. The test yields a negative result in 180 individuals and a positive result in 20. What are the specificity and the false positive rate of this test?

• ASpecificity = 10%, False positive rate = 90%.
• BSpecificity = 80%, False positive rate = 20%.
• CSpecificity = 90%, False positive rate = 10%.
• DSpecificity = 95%, False positive rate = 5%.

A screening test with 95% sensitivity and 95% specificity is applied to a population of 10,000 individuals where the disease prevalence is 1% (100 diseased, 9,900 healthy). What is the positive predictive value (PPV) of a positive test result?

• A~16.1% (approximately 1 in 6 positive results is a true positive).
• B95.0% (equal to the test sensitivity).
• C50.0%.
• D90.0%.

In statistical hypothesis testing, what defines a Type I error (alpha) and what is the statistical power of a study?

• AType I error is failing to reject a false null hypothesis; power is the significance leve…
• BType I error is accepting both hypotheses simultaneously; power is the standard deviation…
• CType I error is incorrectly accepting the null hypothesis; power is 1 - alpha.
• DType I error is rejecting the null hypothesis (H0) when it is actually true; statistical…

In human anatomical terminology, which reference plane passes vertically through the midline of the body to divide it into symmetrical right and left halves?

• ACoronal (frontal) plane.
• BMedian (midsagittal) plane.
• CHorizontal (transverse / axial) plane.
• DOblique subcostal plane.

What is the standard anatomical position of the human body used as the universal reference for all anatomical descriptions?

• ABody standing erect, eyes looking forward to the horizon, upper limbs at the sides with p…
• BBody lying supine on an examination table with arms crossed over the chest and feet plant…
• CBody standing upright with upper limbs abducted 90 degrees and palms facing backward (pro…
• DBody in the lateral decubitus position with knees flexed 90 degrees and neck rotated.

Anatomically, how are bones classified based on their macroscopic shapes and dimensions, and which of the following is a classic example of a flat bone?

• ALong bone; humerus.
• BShort bone; talus.
• CFlat bone; scapula (or cranial vault bone).
• DSesamoid bone; femur.

What are the histological and physiological properties of the periosteum covering the external surface of living bones?

• AIt is an avascular fibrous layer that covers only the articular cartilaginous surfaces.
• BIt consists of a single layer of osteoclasts that continuously breaks down cortical bone…
• CIt is an internal membranous lining that lines the medullary cavity and trabecular meshwo…
• DIt is a dense, highly vascularized and sensory-innervated fibrous membrane with an inner…

Which developmental mechanism characterizes endochondral ossification, as observed in the formation and growth of long bones?

• AMesenchymal cells condense into a hyaline cartilage model, which is subsequently calcifie…
• BMesenchymal cells differentiate directly into osteoblasts within embryonic fibrous connec…
• COsteoclasts synthesize osteoid directly in the blood without cellular precursors.
• DChondrocytes convert directly into skeletal muscle fibers under the influence of calciton…

How many vertebrae compose each anatomical region of the human vertebral column, and what are the primary physiological sagittal curvatures present at birth?

• A8 cervical, 10 thoracic, 6 lumbar, and 5 fused sacral; cervical and lumbar lordoses are p…
• B7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, and 4 fused coccygeal; thoracic and sa…
• C6 cervical, 12 thoracic, 6 lumbar, and 3 sacral; all sagittal curves develop postnatally…
• D7 cervical, 14 thoracic, 4 lumbar, and 1 single coccygeal; lordoses are primary and kypho…

Which unique anatomical feature distinguishes the first two cervical vertebrae (atlas, C1 and axis, C2) from typical subaxial cervical vertebrae?

• AThe atlas possesses a massive vertebral body and a bifid spinous process, while the axis…
• BNeither the atlas nor the axis has articular facets or arches.
• CThe atlas lacks a vertebral body and a spinous process, while the axis possesses a tooth-…
• DThe axis has no pedicles or laminae and fuses completely with the occipital condyles in e…

In the human thoracic wall, what is the anatomical arrangement of the intercostal neurovascular bundle located within the costal groove along the inferior margin of each rib?

• AFrom superior to inferior: Nerve, Artery, Vein (NAV).
• BFrom superior to inferior: Artery, Vein, Nerve (AVN).
• CFrom superior to inferior: Nerve, Vein, Artery (NVA).
• DFrom superior to inferior: Vein, Artery, Nerve (VAN).

Which structural class of joints permits free movement, features a fluid-filled joint cavity, and is lined by a synovial membrane enclosed within a fibrous capsule?

• ASynovial joints (diarthroses).
• BFibrous joints (synarthroses).
• CCartilaginous joints (amphiarthroses).
• DSyndesmoses.