Iran Medical Basic Sciences Exam key terms

The standard reference posture: standing upright, face forward, arms at the sides with palms facing forward, and feet flat and parallel. All directional terms such as medial, lateral, proximal, and distal are defined relative to this posture.

Imaginary flat surfaces used to section the body: sagittal (dividing left and right), coronal or frontal (dividing anterior and posterior), and transverse or horizontal (dividing superior and inferior). Oblique planes cut at any other angle.

Directional terms describing distance along a limb from its attachment to the trunk. Proximal means nearer the trunk or point of origin; distal means farther from it. They are not interchangeable with superior and inferior, which refer to the whole body axis.

Covering tissue classified by cell layers (simple versus stratified) and surface cell shape (squamous, cuboidal, columnar). Simple epithelia suit absorption and diffusion; stratified epithelia resist friction. Pseudostratified and transitional forms are special categories with distinct functions.

A thin extracellular layer beneath epithelial cells, composed of a basal lamina (laminin, type IV collagen) and reticular lamina. It anchors epithelium to connective tissue, filters molecules, and guides repair after injury. Its disruption permits carcinoma invasion.

A unicellular mucin-secreting cell scattered in respiratory and intestinal epithelium. Apical mucinogen granules discharge to form mucus that lubricates and protects the surface. Chronic irritation causes goblet cell hyperplasia, a feature of chronic bronchitis.

Formation of the embryonic central nervous system. The neural plate folds into a groove and closes into a tube; the cranial end becomes brain, the caudal end spinal cord. Failure of closure causes defects such as anencephaly or spina bifida, reduced by periconceptional folic acid.

Each of the five arches contributes skeletal, muscular, and nerve elements. Arch one gives mandible, malleus, incus, and trigeminal nerve; arch two gives stapes and facial nerve; arch three gives hyoid and glossopharyngeal nerve. Arch four and six supply laryngeal cartilages and vagal branches.

The volume of blood ejected by one ventricle per minute, equal to heart rate multiplied by stroke volume. Typical adult resting value is about five litres per minute. It rises in exercise through both rate and stroke volume and falls in shock or heart failure.

The principle that increasing ventricular filling (preload) stretches cardiac muscle and increases the force of contraction, raising stroke volume. It matches output to venous return. Excessive dilation, as in heart failure, flattens the curve and reduces effectiveness.

The volume of blood ejected by one ventricle per beat, determined by preload, afterload, and contractility. Resting adult value is roughly 70 millilitres. It falls when afterload rises or contractility falls, and rises with greater venous return.

The average arterial pressure over the cardiac cycle, approximately diastolic pressure plus one third of pulse pressure. Because systole lasts about one third of the cycle at rest, it lies closer to diastolic than to systolic pressure. It determines organ perfusion.

The volume of plasma filtered by both kidneys per minute, normally about 125 millilitres. It depends on net filtration pressure, which reflects hydrostatic and oncotic gradients across the glomerular capillary, and on the filtration coefficient. It is the best overall index of kidney function.

The volume of plasma from which a substance is completely removed per unit time. It is calculated as urine concentration times urine flow divided by plasma concentration. Inulin clearance measures glomerular filtration rate; para-aminohippurate clearance estimates renal plasma flow.

A model of enzyme behaviour in which reaction velocity rises with substrate concentration and approaches a maximum, Vmax, at saturation. The Michaelis constant Km equals the substrate concentration at half-maximal velocity and reflects enzyme affinity; a low Km indicates high affinity.

Reversible inhibition in which an inhibitor resembling the substrate binds the active site. It raises apparent Km without changing Vmax, because excess substrate can displace the inhibitor. Statins inhibiting HMG-CoA reductase illustrate this pattern.

Non-protein molecules required for enzyme activity. Metal ions such as zinc and magnesium are inorganic cofactors; coenzymes such as NAD+, FAD, and coenzyme A are organic and often vitamin-derived. Deficiency of the parent vitamin impairs the reactions that depend on them.

The cytoplasmic pathway converting glucose to pyruvate, regulated at three irreversible steps: hexokinase or glucokinase, phosphofructokinase-1, and pyruvate kinase. Phosphofructokinase-1 is the key control point, inhibited by ATP and citrate and activated by AMP and fructose-2,6-bisphosphate.

Synthesis of glucose from non-carbohydrate precursors such as lactate, glycerol, and amino acids, occurring mainly in liver and kidney cortex. It uses four bypass reactions to reverse the irreversible steps of glycolysis. It sustains blood glucose during fasting.

Criteria linking a microorganism to a disease: the organism is found in all cases, can be isolated in pure culture, reproduces disease when inoculated into a susceptible host, and is recovered from the experimentally infected host. They fail for unculturable and purely toxigenic agents.

A heat-labile protein secreted by living bacteria, often highly potent and specific in action. Examples include diphtheria toxin, tetanus toxin, and cholera toxin. Toxoids made by chemical inactivation are used as vaccines because antibody neutralises the toxin.

Lipopolysaccharide in the outer membrane of Gram-negative bacteria, released on cell lysis. Lipid A triggers fever, hypotension, and disseminated intravascular coagulation through toll-like receptor 4. It is heat-stable and not converted into a toxoid.

The resident microorganisms colonising skin and mucous membranes without causing disease in the healthy host. They compete with pathogens for nutrients and attachment sites, produce antimicrobial substances, and support immune maturation. Broad-spectrum antibiotics can disrupt them and permit overgrowth.

Infection by an organism that rarely harms immunocompetent hosts but causes disease when defences are impaired. Examples include Pneumocystis jirovecii in HIV infection and Aspergillus in neutropenia. The causative agent often comes from the patient's own flora or the environment.

Display of peptide fragments on cell-surface MHC molecules to T lymphocytes. MHC class I presents endogenous peptides to CD8 cytotoxic cells; MHC class II presents exogenous peptides to CD4 helper cells. This recognition initiates adaptive immune responses.

A gene cluster encoding molecules that present peptides to T cells and determine transplant compatibility. Human leukocyte antigen genes are highly polymorphic. Class I molecules occur on nearly all nucleated cells; class II molecules are restricted mainly to antigen-presenting cells.

Five antibody isotypes with distinct roles. IgM is the first made in infection and is pentameric; IgG predominates, crosses the placenta, and fixes complement; IgA protects mucosae as a dimer; IgE mediates allergy; IgD acts mainly as a B-cell receptor.

Protection acquired by transfer of antibodies or cells from another individual, giving immediate but temporary defence without memory. Examples are maternal IgG across the placenta, colostral IgA, and immunoglobulin therapy. It differs from active immunity, which requires the host response.

A vaccine made from weakened organisms that replicate in the host and mimic natural infection, producing strong humoral and cellular immunity, often with one or two doses. It is contraindicated in pregnancy and severe immunodeficiency because replication may cause disease.

Reversible replacement of one mature epithelial type by another better suited to stress, as when bronchial columnar epithelium becomes squamous in smokers. It is an adaptation, not malignancy, but the new epithelium may predispose to dysplasia and cancer.

Disordered growth in which cells show variation in size and shape, hyperchromatic nuclei, and loss of normal orientation, but the basement membrane remains intact. It is not yet invasive cancer, though severe dysplasia is a direct precursor. Grading reflects how much epithelium is affected.

Programmed cell death in which caspases dismantle the cell into membrane-bound bodies cleared by phagocytes without inflammation. It is energy-dependent and tightly regulated by Bcl-2 family proteins. It removes damaged or unwanted cells, contrasting with the swelling and rupture of necrosis.

Additional tissue damage caused when blood flow returns to ischaemic tissue. Restored oxygen generates reactive species, activates complement, and recruits neutrophils, worsening injury beyond that of ischaemia alone. It complicates infarction, transplantation, and reperfusion therapy.

The apparent volume relating the amount of drug in the body to its plasma concentration. A large value indicates extensive tissue binding or distribution, as with digoxin; a small value indicates confinement to plasma, as with warfarin. It determines loading dose.

The time required for plasma drug concentration to fall by half, determined by volume of distribution and clearance. It guides dosing interval and predicts time to steady state, reached after about four to five half-lives. Disease altering clearance or volume changes it.

Presystemic elimination of an orally administered drug by gut wall and liver before it reaches systemic circulation, reducing bioavailability. Drugs with extensive first-pass loss, such as propranolol and morphine, need higher oral doses or alternative routes. Grapefruit juice can inhibit intestinal metabolism and raise levels.

The ratio of a drug dose producing toxicity to the dose producing the desired effect, often estimated as toxic divided by effective dose. A narrow index, as with digoxin, lithium, and warfarin, demands therapeutic drug monitoring. It reflects the margin of safety.

Characteristic distributions of a trait in families. Autosomal dominant appears in every generation with male and female affected equally; autosomal recessive affects siblings with unaffected parents; X-linked recessive affects mainly males through carrier mothers. Mitochondrial traits pass through the maternal line.

The principle that allele and genotype frequencies in a large, randomly mating population remain constant without mutation, migration, or selection. For a two-allele locus, genotype frequencies are p squared, 2pq, and q squared. Deviations indicate evolutionary forces or genotyping error.

Production of the same clinical phenotype by different genetic mechanisms. Locus heterogeneity means mutations at different genes, as in osteogenesis imperfecta; allelic heterogeneity means different mutations within one gene, as in cystic fibrosis. It complicates molecular testing strategies.

Sensitivity is the proportion of truly diseased people correctly identified by a test; specificity is the proportion of truly non-diseased people correctly classified. They are intrinsic properties of the test and independent of prevalence, unlike predictive values.

The probability that a person with a positive test result truly has the disease. It depends on test sensitivity and specificity plus disease prevalence, rising as prevalence rises. It is therefore lower in screening programmes for rare conditions.

Distortion of an exposure-outcome association by a third variable related to both, which is not on the causal pathway. Age and smoking confound many associations. Randomisation, restriction, matching, and multivariable adjustment address it; stratification can reveal it.

The ratio of disease incidence in exposed individuals to incidence in unexposed individuals, used in cohort studies. A value of one indicates no association; above one indicates increased risk. It differs from the odds ratio used in case-control studies.

Apparent improvement in survival caused only by earlier diagnosis, which lengthens the measured interval from detection to death without postponing death. Screening programmes are especially prone to it. Mortality, not survival time, is the reliable outcome measure.

A patient's voluntary authorisation of a medical intervention after receiving adequate information about its nature, purpose, benefits, alternatives, and material risks, and after demonstrating understanding. It presumes decision-making capacity and may be withdrawn at any time.

The duty to keep information learned in a professional relationship private, disclosing it only with consent or where law permits or requires, such as notifiable disease reporting or serious risk to others. It underpins trust and is a core element of professional codes.

A clinical judgement that an intervention cannot achieve its intended physiological goal or any meaningful benefit for the patient. It is not a licence to abandon care; comfort measures continue. Where disputes arise, ethics consultation and fair process are preferred to unilateral decisions.

A water-soluble B vitamin (B9) required for one-carbon transfer in nucleotide synthesis and homocysteine remethylation. Deficiency causes macrocytic anaemia and is teratogenic, so periconceptional supplementation prevents neural tube defects. It is abundant in leafy greens and legumes.

Weight in kilograms divided by height in metres squared, used to classify adult nutritional status. Values of 25 or more indicate overweight and 30 or more obesity, with lower thresholds for some Asian populations. It does not distinguish fat from muscle or reflect fat distribution.

Indirect protection of susceptible people when a sufficiently high proportion of a population is immune, interrupting transmission chains. The threshold rises with the infectiousness of the pathogen. It protects those who cannot be vaccinated, such as infants and immunocompromised patients.