USMLE Step 1 cheat sheet

Henderson–Hasselbalch: pH = 6.1 + log([HCO3−] / (0.03 × PaCO2)); an acute 10 mmHg rise in PaCO2 lowers pH by roughly 0.08.

Competitive inhibitors raise Km with unchanged Vmax; noncompetitive inhibitors lower Vmax with unchanged Km; uncompetitive inhibitors lower both.

Hardy–Weinberg: p + q = 1 and p² + 2pq + q² = 1; the carrier frequency for a recessive allele equals 2pq.

Mitochondrial DNA is inherited maternally; an affected father transmits to no children, while an affected mother places all children at risk.

Male-to-male transmission excludes X-linked inheritance and favors autosomal dominant or Y-linked patterns.

Type I collagen: bone, skin, tendon, dentin; type II: cartilage; type III: blood vessels and uterus; type IV: basement membrane.

Lysosomal storage diseases are autosomal recessive except Fabry disease and Hunter syndrome, which are X-linked.

Cell-cycle checkpoints: p53 halts the cycle at G1/S after DNA damage; cyclin D–CDK4/6 and cyclin E–CDK2 drive the G1/S transition.

Km is the substrate concentration at half-maximal velocity; a low Km indicates high enzyme affinity for substrate.

Broca area (inferior frontal gyrus) lesions impair speech production with preserved comprehension; Wernicke area (superior temporal gyrus) lesions produce fluent speech with impaired comprehension.

ACA stroke: contralateral leg weakness greater than face/arm; MCA stroke: contralateral face/arm weakness greater than leg, plus aphasia if the dominant hemisphere is involved.

Motor cortex somatotopy: the leg is represented medially near the longitudinal fissure; face and hand are lateral near the sylvian fissure.

Cerebellar hemisphere lesions cause ipsilateral ataxia, intention tremor, and dysdiadochokinesia; vermal lesions cause truncal ataxia and wide-based gait.

Dopamine pathways: nigrostriatal (movement; lost in Parkinson disease), mesolimbic (reward), mesocortical, and tuberoinfundibular (tonically inhibits prolactin release).

Corneal reflex: afferent limb is V1 (ophthalmic division), efferent limb is VII (facial); the reflex is lost in CN VII palsy and in V1 lesions.

REM sleep shows low-voltage mixed-frequency (beta-like) EEG, skeletal muscle atonia, and dreaming; slow-wave N3 sleep shows delta waves.

Serotonin syndrome presents with clonus, hyperreflexia, hyperthermia, and agitation after serotonergic agents such as SSRIs combined with MAO inhibitors.

Sympathetic preganglionic neurons release acetylcholine onto nicotinic receptors; postganglionic sympathetic fibers release norepinephrine except at sweat glands, which use acetylcholine.

Iron deficiency: low ferritin, high TIBC, low transferrin saturation, high RDW; anemia of chronic disease: low TIBC with normal or high ferritin.

B12 deficiency raises both methylmalonic acid and homocysteine; folate deficiency raises homocysteine only.

Beta-thalassemia shows elevated HbA2 and HbF; alpha-thalassemia with deletion of all four alpha genes produces Hb Bart's hydrops fetalis.

von Willebrand factor binds platelet GPIb for adhesion; fibrinogen binds GPIIb/IIIa for aggregation; aspirin irreversibly acetylates COX-1.

C3 deficiency predisposes to recurrent pyogenic infections; terminal complement (C5–C9) deficiency predisposes to recurrent Neisseria bacteremia.

CD8+ T cells recognize antigen presented on MHC I; CD4+ T cells recognize antigen presented on MHC II.

G6PD deficiency is X-linked; oxidative stress produces Heinz bodies, bite cells, and intravascular hemolysis.

Type O red cells are the universal donor and type AB the universal recipient; type AB plasma is the universal donor plasma.

Fetal hemoglobin has higher oxygen affinity than adult hemoglobin because it binds 2,3-BPG poorly.

CO = HR × SV; MAP = CO × TPR; MAP is approximated as DBP + one-third of the pulse pressure.

Ejection fraction = SV/EDV; normal EF is at least 55%; HFrEF is defined by EF of 40% or less.

JVP waves: a = atrial contraction, c = tricuspid bulging into the right atrium, v = atrial filling; the a wave is absent in atrial fibrillation.

Aortic stenosis: crescendo–decrescendo systolic murmur radiating to the carotids with pulsus parvus et tardus.

Mitral regurgitation: holosystolic murmur radiating to the axilla; mitral stenosis: opening snap followed by a diastolic rumble.

ECG intervals: PR 0.12–0.20 s and QRS under 0.12 s; a prolonged PR defines first-degree AV block.

Septic shock is distributive with warm skin, wide pulse pressure, and low systemic vascular resistance; cardiogenic shock has cold skin, narrow pulse pressure, and high resistance.

Renin is released by juxtaglomerular cells in response to low renal perfusion, reduced distal NaCl delivery, and β1 stimulation; angiotensin II is a potent vasoconstrictor.

BNP is released by ventricular stretch in heart failure; levels also rise with advanced age and renal impairment.

Insulin from pancreatic β cells lowers glucose by translocating GLUT4; glucagon from α cells raises glucose through glycogenolysis and gluconeogenesis.

Graves disease: low TSH, high free T4, positive TSH receptor antibodies; propylthiouracil also inhibits peripheral 5′-deiodinase, reducing T4-to-T3 conversion.

Primary hyperaldosteronism: high aldosterone with suppressed renin, hypertension, hypokalemia, and metabolic alkalosis.

Primary adrenal insufficiency (Addison): low cortisol, elevated ACTH, hyperpigmentation, hyperkalemia, and hyponatremia.

PTH raises serum calcium by increasing bone resorption and distal tubular reabsorption, and lowers phosphate by increasing renal phosphate excretion.

MEN 2A: medullary thyroid carcinoma, pheochromocytoma, and primary hyperparathyroidism, caused by activating RET mutations.

HbA1c reflects mean glycemia over the preceding 2–3 months; DKA shows an anion gap metabolic acidosis with β-hydroxybutyrate.

Cushing syndrome screening: 24-hour urine free cortisol, late-night salivary cortisol, or the 1 mg overnight dexamethasone suppression test.

In type 1 diabetes, insulin deficiency permits unchecked lipolysis and ketogenesis; in hyperosmolar hyperglycemic state, residual insulin suppresses ketones but not hyperglycemia.

Liver acinar zones: zone 1 is periportal with the highest oxygen tension; zone 3 is centrilobular with the highest CYP450 activity and greatest susceptibility to ischemia and toxins.

Unconjugated bilirubin is albumin-bound and not water soluble; conjugated bilirubin is water soluble and can appear in urine.

Celiac disease: IgA anti-tissue transglutaminase antibodies, villous atrophy with crypt hyperplasia, and HLA-DQ2 or HLA-DQ8.

Duodenal ulcers typically improve with eating; gastric ulcers worsen with eating and require biopsy to exclude malignancy.

Cholecystokinin causes gallbladder contraction and pancreatic enzyme secretion; secretin stimulates pancreatic bicarbonate secretion.

Vitamin B12 requires intrinsic factor for absorption in the terminal ileum; ileal resection produces macrocytic anemia.

Acute pancreatitis: lipase is more specific than amylase; gallstones and alcohol are the leading causes.

Right-sided colon cancer presents with iron-deficiency anemia; left-sided lesions present with obstruction, narrowing, and hematochezia.

Type I muscle fibers are slow, oxidative, and fatigue resistant; type II fibers are fast, glycolytic, and fatigue quickly.

Osteoblasts build bone and express RANKL; osteoclasts resorb bone; osteoprotegerin acts as a decoy receptor that blocks RANKL.

Fracture healing sequence: hematoma formation, fibrocartilaginous soft callus, bony hard callus, then remodeling.

Compartment syndrome: pain with passive stretch, pallor, paresthesia, paralysis, and late pulselessness; compartment pressure above 30 mmHg indicates fasciotomy.

Duchenne muscular dystrophy is X-linked recessive with absent dystrophin; Gower sign and calf pseudohypertrophy appear in early childhood.

Osteoporosis is defined by a DXA T-score of −2.5 or lower; osteomalacia and rickets show low calcium and phosphate with elevated alkaline phosphatase and PTH.

Articular cartilage is avascular and aneural and consists mainly of type II collagen and proteoglycans.

Sarcomere contraction: calcium binds troponin C, tropomyosin shifts, actin–myosin cross-bridges form, and ATP is required for both contraction and relaxation.

Epidermal layers deep to superficial: basale, spinosum, granulosum, lucidum (palms and soles only), corneum.

Melanocytes derive from neural crest; keratinocytes form the bulk of the epidermis and produce keratin.

Psoriasis: parakeratosis, regular acanthosis, Munro microabscesses in the stratum corneum, and the Auspitz sign on removal of scale.

Pemphigus vulgaris: intraepidermal flaccid bullae with anti-desmoglein 3 antibodies and a positive Nikolsky sign; bullous pemphigoid: subepidermal tense bullae with anti-hemidesmosomal antibodies.

Basal cell carcinoma shows peripheral palisading and peritumoral clefting; squamous cell carcinoma shows keratin pearls and intercellular bridges.

Melanoma prognosis is driven by Breslow depth and ulceration; ABCDE comprises asymmetry, border irregularity, color variation, diameter, and evolution.

Rule of nines in adults: each arm 9%, each leg 18%, anterior trunk 18%, posterior trunk 18%, head 9%, perineum 1%.

Acne vulgaris involves Cutibacterium acnes colonization of sebaceous follicles with androgen-driven sebum production.

Clearance = (urine concentration × urine flow) / plasma concentration; inulin clearance measures GFR and PAH clearance measures renal plasma flow.

Filtration fraction = GFR / renal plasma flow and is normally about 20%; it rises with increased efferent arteriolar tone.

The proximal tubule reabsorbs about 65% of filtered sodium and glucose via SGLT2; the thick ascending limb uses the Na–K–2Cl cotransporter and is impermeable to water.

Furosemide inhibits NKCC2 in the thick ascending limb; thiazides inhibit the distal convoluted tubule Na–Cl cotransporter; spironolactone blocks aldosterone receptors.

ADH inserts aquaporin-2 channels in collecting duct principal cells; aldosterone increases ENaC activity and basolateral Na/K-ATPase.

Anion gap = Na − (Cl + HCO3), normally 8–12 mEq/L; an elevated gap suggests ketoacidosis, lactic acidosis, uremia, or toxins.

FENa below 1% favors prerenal azotemia and above 2% favors acute tubular necrosis; FENa is unreliable in patients receiving diuretics.

Renal artery stenosis raises renin and angiotensin II, producing hypertension and hypokalemia; ACE inhibitors can precipitate acute kidney injury in bilateral stenosis.

Leydig cells respond to LH and produce testosterone; Sertoli cells respond to FSH and produce inhibin and androgen-binding protein.

The LH surge occurs about 24–36 hours before ovulation (cycle day 14 in a 28-day cycle); the corpus luteum then secretes progesterone.

Estrogen drives endometrial proliferation in the follicular phase; progesterone produces secretory change in the luteal phase.

hCG is produced by syncytiotrophoblasts and maintains the corpus luteum; it shares its alpha subunit with LH, FSH, and TSH.

Primary oocytes remain arrested in prophase I of meiosis until the LH surge triggers completion of meiosis I.

Müllerian ducts form the uterus, fallopian tubes, and upper vagina; Wolffian ducts form the epididymis, vas deferens, and seminal vesicles under testosterone.

Turner syndrome (45,X): streak ovaries, short stature, webbed neck, and coarctation risk; Klinefelter syndrome (47,XXY): tall stature, gynecomastia, azoospermia.

Antimüllerian hormone from Sertoli cells causes regression of the Müllerian ducts in the male fetus.

Compliance = Δvolume / Δpressure; surfactant from type II pneumocytes lowers alveolar surface tension and prevents collapse (Law of Laplace: P = 2T/r).

Oxygen content = (1.34 × Hb × SaO2) + (0.003 × PaO2); dissolved oxygen contributes only a small fraction of total content.

Right shift of the oxyhemoglobin dissociation curve (increased CO2, H+, temperature, or 2,3-BPG) reduces affinity and enhances tissue oxygen unloading.

CO2 is transported mainly as bicarbonate (about 70%), with 20–23% bound to hemoglobin as carbaminohemoglobin and about 7% dissolved.

Obstructive disease: FEV1/FVC below 0.70 with increased residual volume; restrictive disease: reduced FVC with a normal or increased ratio.

Centriacinar emphysema is associated with smoking; panacinar emphysema is associated with α1-antitrypsin deficiency and shows basilar predominance.

Asthma shows Curschmann spirals, Charcot–Leyden crystals, and a thickened basement membrane; COPD shows goblet cell hyperplasia and airway fibrosis.

Shunt (V/Q = 0) is perfusion without ventilation and does not correct with supplemental oxygen; dead space (V/Q approaching infinity) is ventilation without perfusion.