USMLE Step 1 flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Michaelis constant Km

Substrate concentration at which an enzyme reaches half of its maximal velocity; a low Km indicates high enzyme affinity for substrate.

Hardy-Weinberg carrier frequency

For a recessive allele with frequency q, heterozygote carriers occur at frequency 2pq, which approximates 2q when q is small.

Competitive inhibition kinetics

A reversible inhibitor competes with substrate at the active site, raising Km while leaving Vmax unchanged because saturating substrate can outcompete the inhibitor.

Broca aphasia

Damage to the inferior frontal gyrus of the dominant hemisphere causes nonfluent speech with preserved comprehension; patients are aware of the deficit and often frustrated.

Nigrostriatal pathway degeneration

Loss of dopaminergic neurons in the substantia nigra pars compacta removes excitatory input to the striatum, producing tremor, rigidity, and bradykinesia in Parkinson disease.

REM sleep features

Rapid eye movements, low-voltage mixed-frequency EEG, skeletal muscle atonia, and vivid dreaming; cycles recur every 90 minutes and lengthen toward morning.

Iron deficiency indices

Low serum ferritin, low transferrin saturation, and elevated total iron-binding capacity; ferritin is the most specific single test and is normal or high in anemia of chronic disease.

B12 versus folate deficiency

B12 deficiency raises methylmalonic acid and homocysteine with neurologic signs; folate deficiency raises homocysteine only, and folate repletion can worsen B12 neurologic damage.

GPIb and GPIIb/IIIa

GPIb binds von Willebrand factor for platelet adhesion to collagen; GPIIb/IIIa binds fibrinogen for aggregation and is blocked by abciximab, eptifibatide, and tirofiban.

Mean arterial pressure

MAP equals cardiac output times total peripheral resistance, approximated as diastolic pressure plus one-third of the pulse pressure; it drives organ perfusion.

Mitral stenosis findings

Diastolic rumble with an opening snap after S2 and a loud S1; rheumatic heart disease is the leading cause and atrial fibrillation is a common complication.

Cardiogenic versus septic shock

Cardiogenic shock shows cold extremities, narrow pulse pressure, and high systemic vascular resistance; septic shock is distributive with warm skin, wide pulse pressure, and low resistance.

Primary hyperaldosteronism

Adrenal aldosterone overproduction suppresses renin, causing hypertension with hypokalemia and metabolic alkalosis; an adrenal adenoma or bilateral hyperplasia is usually responsible.

Addison disease laboratory

Primary adrenal insufficiency gives low cortisol with elevated ACTH, hyponatremia, hyperkalemia, and hyperpigmentation from increased POMC-derived melanocyte-stimulating hormone.

MEN 2A components

Medullary thyroid carcinoma, pheochromocytoma, and primary hyperparathyroidism caused by activating RET mutations; relatives are screened with RET testing and calcitonin.

Zone 3 hepatic injury

Centrilobular hepatocytes are farthest from the portal supply, have the highest cytochrome P450 activity, and are most vulnerable to ischemia and toxins such as acetaminophen.

Celiac disease serology

IgA anti-tissue transglutaminase is the preferred screen, confirmed by IgA anti-endomysial antibody; biopsy shows villous atrophy and crypt hyperplasia, and HLA-DQ2 or DQ8 is nearly always present.

Secretin and cholecystokinin

Duodenal S cells release secretin in response to acid, stimulating pancreatic bicarbonate; I cells release cholecystokinin in response to fat, causing gallbladder contraction and enzyme secretion.

RANK-RANKL-OPG axis

RANKL on osteoblasts binds RANK on osteoclast precursors to drive differentiation; osteoprotegerin is a decoy receptor, and denosumab is a monoclonal antibody against RANKL.

Duchenne muscular dystrophy

X-linked recessive loss of dystrophin causes progressive proximal weakness, calf pseudohypertrophy, Gower sign, and elevated creatine kinase, with cardiomyopathy and respiratory failure later.

Osteoporosis T-score

A DXA T-score compares bone mineral density with a young healthy adult reference; −2.5 or lower defines osteoporosis and −1.0 to −2.5 defines osteopenia.

Pemphigus versus pemphigoid

Pemphigus vulgaris targets desmoglein 3, giving intraepidermal flaccid bullae and a positive Nikolsky sign; bullous pemphigoid targets hemidesmosomal antigens, giving subepidermal tense bullae.

Psoriasis histology

Acanthosis with regular elongation of rete ridges, parakeratosis, loss of the granular layer, dilated dermal capillaries, and Munro microabscesses of neutrophils in the stratum corneum.

Filtration fraction

The fraction of renal plasma flow filtered at the glomerulus, equal to GFR divided by renal plasma flow and normally about 20%; it rises with increased efferent arteriolar tone.

Fractional excretion of sodium

FENa = (urine Na × plasma creatinine) / (plasma Na × urine creatinine) × 100%; below 1% favors prerenal azotemia and above 2% favors acute tubular necrosis.

Thick ascending limb transport

The Na-K-2Cl cotransporter reabsorbs solute while the segment is impermeable to water, diluting tubular fluid and building the medullary gradient; loop diuretics inhibit this transporter.

Luteinizing hormone surge

A sharp LH rise triggered by positive estrogen feedback on the anterior pituitary precedes ovulation by 24 to 36 hours and induces completion of meiosis I in the oocyte.

Müllerian inhibiting substance

Antimüllerian hormone from fetal Sertoli cells causes regression of the Müllerian ducts, while testosterone from Leydig cells maintains the Wolffian ducts.

Klinefelter syndrome

A 47,XXY karyotype causes tall stature, gynecomastia, small firm testes, azoospermia, and elevated gonadotropins; it is a common cause of male infertility.

Laplace law in alveoli

Alveolar pressure equals twice the surface tension divided by radius, so small alveoli tend to collapse; surfactant lowers surface tension more in small alveoli and stabilizes them.

Physiologic shunt

Perfusion of unventilated lung (V/Q of zero) allows deoxygenated blood into systemic circulation, causing hypoxemia that does not correct with supplemental oxygen.

Obstructive versus restrictive spirometry

Obstruction lowers FEV1 more than FVC, giving FEV1/FVC below 0.70; restriction lowers FVC with a normal or elevated ratio, confirmed by reduced total lung capacity.