6/3 UWorld

Question 1

Mutation and presentation of Williams syndrome

Answer 1

Microdeletion of long arm of chromosome 7 (deletion includes elastin gene)

“Elfin” facies, intellectual disability, hypercalcemia, well-developed verbal skills, extreme friendliess with strangers

THINK: Will Ferrel in the movie elf

Question 2

Presentation of Gardner syndrome

Answer 2

• Familial adenomatous polyposis + tumors outside the colon
• Osteomas / soft tissue tumors
• Supernumerary teeth
  
  • THINK: Gardener from the Kaposi sketchy
    
    • Digging through haustra-shaped planters à FAP
    • Digs up a bone à osteoma
    • Old lady smiles with extra teeth à supernumerary teeth

Question 3

Mutation and presentation of Tuberous sclerosis

Answer 3

• Due to mutated hamartin (or tuberin) gene
  
  • TSC1/TSC2 mutation on chromosome 16
• Triad:
  
  • Angiofibromas, mental retardation, seizures
• Other symptoms: Ash-leaf spots, hamartomas of CNS and skin, angiomyolipoma
• Incomplete penetrance and variable presentation

Question 4

Presentation of VHL disease

Answer 4

Presentation = HARP

• Hemangioblastoma (retina, brainstem, cerebellum)
• Angiomatosis
• bilateral Renal cell carcinoma
• Pheochromocytoma

Question 5

Compare pathogenesis in albinism vs. vitiligo

Answer 5

Albinism = normal melanocyte number with decreased melanin production (due to decreased tyrosinase activity)

Vitiligo = autoimmune destruction of melanocytes

Question 6

What are the branches of the common hepatic artery

Answer 6

• Common hepatic artery
  
  • Gastroduodenal artery
    
    • R gastroepiploic = greater curvature of stomach (anastomoses with L gastroepiploic)
    • Anterior superior pancreaticoduodenal à head of pancreas, proximal duodenum
  • Right gastric artery = lesser curvature of stomach (anastomoses with L gastric a.)
  • Proper hepatic artery

Question 7

Differentiate between case-control study and cohort study

Answer 7

Case control = compare a group with disease to group without disease and look for prior exposures/risk factors (is retrospective)

Cohort study = compare a group with risk factor to a group without risk factor and look for likelihood of disease (can be prosepective or retrospective)

Question 8

Differentiate between incidence and prevalence

Answer 8

• Prevalence = existing cases at a point in time
• Incidence = number of new cases during a specific time period

Question 9

Using this table, what are the formulas for sensitivity, specificity, PPV, and NPV

Answer 9

• Sensitivity = true positive rate
  
  • Proportion of people with the disease who get a positive test result
  • TP / (TP + FN)
• Specificity = true negative rate
  
  • Proportion of people without the disease who get a negative test result
  • TN / (TN + FP)
• Positive predictive value
  
  • Proportion of positive test results that actually have the disease
  • TP / (TP + FP)
• Negative predictive value
  
  • Proportion of negative test results that don’t actually have the disease
  • TN / (TN + FN)

Question 10

Which values (sensitivity, specificity, PPV, NPV), change with disease prevalence

Answer 10

PPV and NPV

• Positive predictive value
  
  • TP / (TP + FP)
  • Increases with increased disease prevalence
    
    • Total # with disease increases, so TP and FN increase
    • Increased TP = increased PPV
• Negative predictive value
  
  • TN / (TN + FN)
  • Decreases with increased disease prevalence
    
    • Total # with disease increases, so TP and FN increase
    • Increased FN = decreased NPV

Question 11

What are the branches of the celiac trunk:

Answer 11

Common hepatic

Splenic

Left gastric

Question 12

What is the equation for incidence

Answer 12

• Incidence = number of new cases during a specific time period
  
  • (# of new cases) / (# of people at risk)

Question 13

Differentiate between precision and accuracy

Answer 13

• Precision
  
  • Reliability of consistency of a test
  • How reproducible is it?
• Accuracy
  
  • Validity of a test
  • How accurate is it?

Question 14

Describe relative risk and its formula based on this table

Answer 14

• Probability of getting disease in exposed group vs. probability of getting disease in unexposed
  
  • Calculated from cohort studies
• Equation:
  
  • RR = [a/(a+b)] / [c/(c+d)]

Question 15

Describe odds ration and its formula given this table

Answer 15

• Odds that group with disease (case) was exposed to a risk factor vs. odds that group without disease (control) was exposed
  
  • Calculated from case-control studies
• Equation:
  
  • OR = [(a/c) / (b/d)] = ad/bc

Question 16

Describe attributable risk and its equation given this table

Answer 16

• Difference in risk among exposed and risk among unexposed
• Equation:
  
  • [a/(a+b)] – [c/(c+d)]

Question 17

Describe absolute risk reduction and its equation given this table

Answer 17

• Looking at how much a beneficial intervention (rather than a risk factor) will affect the risk of disease
  
  • Risk of those not receiving intervention minus risk of those receiving intervention
• So:
  
  • a/(a+b) = people who received intervention and still got disease
  • c/(c+d) = people who did not receive intervention and got the disease
• Equation:
  
  • ARR = [c/(c+d)] - [a/(a+b)]
  • THINK: ARR = NPV – PPV

Question 18

Equation for number needed to treat

Answer 18

• 1/ARR

Question 19

Equation for number needed to harm

Answer 19

1/AR (attributable risk)

Question 20

Describe relative risk reduction and its equation

Answer 20

• The proportion of risk reduction attributable to the intervention as compared to a control
• Equation:
  
  • RRR = 1 – RR
• Example:
  
  • If 2% of people who received a flu vaccine develop the flu vs. 8% of unvaccinated people who develop the flu, then RR = 2/8 = 0.25, and RRR = 0.75

Question 21

What is it called when a study population is chosen from the hospital

Answer 21

Berkson bias

Question 22

What is non-response bias

Answer 22

Nonrespondants meaningfully differ from respondants

Question 23

What is procedure bias?

Answer 23

• Subjects in different groups are not treated the same

Question 24

What is the Pygmalion effect

Answer 24

• Self-fulfilling prophecy
• Researcher’s belief in treatment changes the outcome

Question 25

What is it called when study subject change their behavior because they know they are being observed

Answer 25

Hawthorne effect

Question 26

What is a crossover study

Answer 26

When patients serve as their own control

Question 27

In a positive skew, which side is the tail, and rank mean median and mode in order from greatest to least

Answer 27

* Longer tail on the right * Mean \> median \> mode

Question 28

What is the null hypothesis (H0) vs. alternative hypothesis (H1)

Answer 28

* Null hypothesis (H0) * States that there is no association between exposure and outcome * Alternative hypothesis (H1) * States that there is an association between exposure and outcome

Question 29

What does a P-value tell you? Does a p-value \< 0.05 mean you accept or rejec the H0?

Answer 29

* Probability that the study results occurred by chance alone, given that the null hypothesis is true * In other words, how compatible is study data with the null hypothesis * Small P-value means that it is unlikely that results occurred by chance alone, and thus you can most likely reject the null hypothesis * P \< 0.05 * Means you can reject the null hypothesis and accept the alternative hypothesis * It means there is a relationship between exposure and outcome * “Statistically significant”

Question 30

Differentiate between Type I (alpha) and Type II (beta) error

Answer 30

* Type I error (alpha) * False-positive error * Incorrect rejection of the null hypothesis * Investigator says there is an association between exposure and outcome, when in reality there is none * Type II error (beta) * False-negative error * Incorrect acceptance of the null hypothesis * Investigator says there is no association between exposure and outcome, when in reality there is

Question 31

What does "power" refer to?

Answer 31

Probability that the test will correctly reject the null hypothesis, when it is indeed false

Question 32

What is the formula for confidence interval

Answer 32

* CI = range from [mean – Z(SEM)] to [mean + Z(SEM)] * SEM = standard error of mean * SEM = standard deviation / square root of sample size (n) * For 90% CI, Z = 1.64 * For 95% CI, Z = 1.96 * For 99% CI, Z = 2.58 * As you increase your confidence level, you widen your confidence interval * As you decrease sample size, you widen your confidence interval

Question 33

What is the test used to check for differences between the _means_ of 2 groups with continous variables

Answer 33

* T-test * THINK: Tea is meant for 2

Question 34

What is the test used to check for the difference between _means_ of 3 or more groups with continuous variables

Answer 34

* ANOVA: * THINK: 3 words in “Analysis Of Variance”

Question 35

What is the test used to check for the difference between 2 or more percentages or proportions of _categorical_ outcomes (not means)

Answer 35

* Chi-square: * THINK: Chi-tegorical

Question 36

Which system, HMO or PPO, requires primary care referrals in order to see specialists + only includes "in-network" physicians

Answer 36

HMO - Health maintencance organization * Preferred Provider Organization (PPO) * No gatekeeper to the specialist * Patient can see whomever they want; however, the cost is higher for “out-of-network” medical care * More expensive than HMO

Question 37

Medicare vs. Medicaid

Answer 37

Medicare = elderly + those with certain conditions and end-stage disease Medicaid = low income

Question 38

Mutation in familal adenomatous polyposis (FAP)

Answer 38

Due to mutation of APC tumor suppressor on chromosome 5q

Question 39

What is the defect in Li-Fraumeni syndrome

Answer 39

Defective p53 (tumor suppressor) - multiple malignancies at an early age

Question 40

Presentation of tuberous sclerosis

Answer 40

* Triad: Angiofibromas, mental retardation, seizures * Other symptoms: Ash-leaf spots, hamartomas of CNS and skin, angiomyolipoma, rhabdomyoma (cardiac tumor)