USMLE Step 1 Review

111 flashcards by MentisLibrary

Original study items across the Step 1 content outline: general principles and biostatistics, biochemistry and genetics, microbiology and immunology, pharmacology, and organ system physiology and pathology.

What do the mnemonics SNOUT and SPIN mean?
A highly SeNsitive test, when Negative, rules a disease OUT. A highly SPecific test, when Positive, rules it IN.
How does disease prevalence affect predictive values?
Higher prevalence raises positive predictive value and lowers negative predictive value.
When is an odds ratio a good approximation of relative risk?
When the outcome is rare in the population studied.
How is number needed to treat calculated?
As the reciprocal of the absolute risk reduction.
Confounding vs effect modification: what is the difference?
A confounder distorts the observed association and should be controlled for; an effect modifier means the true effect genuinely differs across strata and should be reported separately.
Define type I error, type II error and power.
Type I error is rejecting a true null hypothesis; type II error is failing to reject a false null hypothesis; power is one minus the probability of a type II error.
What are lead-time bias and length-time bias?
Lead-time bias makes survival appear longer simply because diagnosis was earlier. Length-time bias makes screening look better because slowly progressive disease is preferentially detected.
Which biases threaten case-control studies in particular?
Recall bias and selection bias.
Incidence vs prevalence: how do they relate?
Incidence is new cases per population at risk per unit time; prevalence is existing cases over the total population. Prevalence approximates incidence multiplied by average disease duration.
Which minors can consent to their own medical care in most jurisdictions?
Emancipated minors, and minors seeking care for sexually transmitted infections, contraception, pregnancy related care and substance use treatment in many jurisdictions.
Which enzyme is rate limiting for glycolysis, and what regulates it?
Phosphofructokinase-1, activated by fructose-2,6-bisphosphate and AMP and inhibited by ATP and citrate.
Why do red blood cells hemolyze under oxidative stress in G6PD deficiency?
Glucose-6-phosphate dehydrogenase generates NADPH, which regenerates reduced glutathione. Without it, oxidized hemoglobin precipitates.
What causes von Gierke disease, and how does it present?
Glucose-6-phosphatase deficiency, giving glycogen storage disease type I with severe fasting hypoglycemia and hepatomegaly.
Which glycogen storage disease causes cardiomegaly in infancy?
Pompe disease, type II, from deficiency of lysosomal acid alpha-glucosidase.
What is the second wind phenomenon, and which disease shows it?
Improvement in exercise tolerance after several minutes of activity, seen in McArdle disease from myophosphorylase deficiency.
Which enzyme deficiency causes Lesch-Nyhan syndrome?
Hypoxanthine-guanine phosphoribosyltransferase, or HGPRT.
How is ornithine transcarbamylase deficiency distinguished from hereditary orotic aciduria?
Both raise urinary orotic acid, but ornithine transcarbamylase deficiency causes hyperammonemia without megaloblastic anemia, while orotic aciduria causes megaloblastic anemia without hyperammonemia.
Prader-Willi vs Angelman syndrome: what is the genetic distinction?
Both involve chromosome 15q11-13, but Prader-Willi results from loss of the paternal contribution and Angelman from loss of the maternal contribution.
What is genetic anticipation, and which diseases show it?
Increasing severity or earlier onset in successive generations, seen in trinucleotide repeat disorders such as Huntington disease and myotonic dystrophy.
State the Hardy-Weinberg equations and their assumptions.
$p + q = 1$ and $p^2 + 2pq + q^2 = 1$, assuming no mutation, no selection, no migration, random mating and a large population.
How does mitochondrial inheritance behave in a pedigree?
Transmission is exclusively maternal, and all children of an affected mother may be affected to varying degrees.
How does homocystinuria differ from Marfan syndrome on examination?
Both are marfanoid, but the lens dislocates downward and inward in homocystinuria and upward and outward in Marfan syndrome.
What causes maple syrup urine disease?
Deficiency of branched-chain alpha-ketoacid dehydrogenase, so leucine, isoleucine and valine accumulate.
How are gram positive cocci separated in the laboratory?
By catalase, which is positive in staphylococci and negative in streptococci, and then coagulase, which is positive in Staphylococcus aureus.
How are streptococci classified by hemolysis and by susceptibility testing?
Alpha hemolytic species split into optochin sensitive and bile soluble Streptococcus pneumoniae and optochin resistant viridans streptococci. Beta hemolytic species split into bacitracin sensitive group A and bacitracin resistant group B.
Exotoxin vs endotoxin: what are the key differences?
Exotoxins are secreted proteins, often extremely potent, heat labile and convertible into toxoid vaccines. Endotoxin is the lipid A of gram negative lipopolysaccharide, heat stable, and cannot be toxoided.
Which two exotoxins inhibit protein synthesis by ADP-ribosylating elongation factor 2?
Diphtheria toxin from Corynebacterium diphtheriae and exotoxin A from Pseudomonas aeruginosa.
Which bacteria are obligate intracellular organisms?
Rickettsia, Chlamydia and Coxiella.
Which organisms are urease positive, and why does it matter?
Proteus, Klebsiella, Helicobacter pylori, Ureaplasma, Nocardia, Cryptococcus and Staphylococcus saprophyticus.
Which organisms are most dangerous in an asplenic patient?
Encapsulated organisms, chiefly Streptococcus pneumoniae, Haemophilus influenzae type b and Neisseria meningitidis.