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USMLE And Medicals
Peter
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Peter
Answering Biostatistics Questions For IFOM And The USMLE: Part 1
~2.9 mins read
Answering Biostatistics questions for IFOM and the USMLE: Part 1
Let us look at the sample NBME question below:
A community public health department has a limited budget for new interventions and must decide between two options. Option A is to reduce exposure to an industrial chemical that increases the risk for leukemia from 0.5 per 100,000/year to 2.0 per 100,000/year. It is estimated that 30% of the working population in the community is exposed to this agent. Option B is to reduce exposure to a different toxin that increases the risk for aplastic anemia from 0.5 per 100,000/year to 50 per 100,000/year. It ¡s estimated that 5% of the working population is exposed to this toxin. The estimated cost of each intervention is US $740,000. It is assumed that each intervention program will have similar effectiveness in eradicating the exposure. The case fatality rates are similar for both diseases. Which of the following is the best rationale for the health department to use in selecting an option?
A) Option A because more fatalities will be prevented
B) Option A because more workers are exposed to the toxic agent
C) Option B because more fatalities will be prevented
D) Option B because more workers are exposed to the toxic agent
E) The best approach cannot be determined based on the information provided
Dissecting the question
- This question targets the topic: Relative Risk [Relative risk is a high yield topic]
- The quick formula for Relative Risk for this purpose is to divide:
Exposed group (or no treatment) / Unexposed group (or with treatment)
- What these kinds of questions ask, is for you to compare the risk of Option A relative to Option B.
- Every other information in the vignette isn’t needed to arrive at the answer.
- You can easily spot this type of question by looking for the keyword “Risk” when you are asked to compare one group to another (Just like the above question).
Solving for an answer:
From Option A (Risk for leukemia):
Exposed group = 2
Unexposed group = 0.5
Putting in the formula:
2/0.5 = 4% (Relative Risk of 4)
From Option B (Risk for aplastic anemia):
Exposed group = 50
Unexposed group = 0.5
Putting in the formula:
50/0.5 = 100% (Relative risk of 100)
So the Risk in Option B is 100% relative to that in Option A which is 4%.
It means it would be better to invest in an intervention that would reduce the risk of disease by 100% than to invest in one that reduces the risk by only 4%.
The answer is "Option C) Option B because more fatalities would be prevented".
It is not “D) Option B because more workers are exposed to the toxic agent” because we are working with the risk increased per year in each option (i.e 100,000/yr), we are not working with the percentages of workers exposed to the agents. The reason is, even though 30% of workers are exposed to Option A’s agent the risk of getting leukemia is 4%. For Option B, only 5% of the working population is exposed to it, but the risk of getting aplastic anemia is 100% from the calculation.
Take home:
When you meet any biostat question that wants you to compare risk in one group to another,
- Remember relative risk
- Extract exposed numbers and unexposed numbers from the questions
- Divide E (exposed) with U (unexposed) [E/U = RR]
- The group with the highest number has the more risk and vice versa.
- Every other information from the vignette may not be helpful
- All the methods or assumptions in these questions might not be perfect but if you grasp the concept, you'll find it easy to answer similar questions.
Reference:
1. Topic: Relative Risk. Book: First Aid for the USMLE step 1 2019. Public Health Sciences/Biostatistics section.
Reference:
1. Topic: Relative Risk. Book: First Aid for the USMLE step 1 2019. Public Health Sciences/Biostatistics section.
Guest Naresh Kumar‎
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Guest Naresh Kumar‎
My USMLE Step 1 Experience
~2.8 mins read
Step 1 Score: 265
Graduated from JSMU, YOG: Dec’17.
Preparation time: 21 Months
Uworld first pass, random: 83.7%
After wrong: 86%
I started my prep in 3rd year but I didn’t give much time to it due to extracurricular activities. I could not even finish Kaplan physiology in 2 months of vacations. Like this I spent my whole 2 years and graduated. The real deal started on January 8th, 2018. It took me 8 months to complete my FA and another 9 months to complete my Uworld. Yes, I am a slow reader. Step 1 has a lot of NBMEs. It took me almost 3 months to complete these assessments after Uworld, and then took my exam on October 9th, 2019.
Resources:
- I used Goljan (just general part), Kaplan(notes and lectures), 100 cases of ethics, Uworld Biostates review, bnb just for extremely selected topics, High yield anatomy(just for upper and lower limbs), pathoma, and of course FA and Uworld. And above all, GOOGLE :P
- Goljan was useful for me. It cleared a lot of concepts of mine. Could not do systems from it because it required a lot of time which I could not afford.
- I did Kaplan and FA side by side. And also noted down points on FA which I felt were important. Kaplan is way more useful if you are weak in basics.
- I did same with Pathoma.
- Started my Uworld on September 2018. Initially my scores were low but it got better with time. I am a slow reader so did less questions a day and no. of question I did varied each day. Noted down Uworld points on my FA so that I can revise it for later.
Assessments:
I took assessments in the following order:
Nbme 7 at 60% Uworld: 15 mistakes
Nbme 11 at 80% Uworld: 19 mistakes
Nbme 12 just after Uworld: 15 mistakes
UWSA 1: 269
Nbme 15: 13 mistakes
Nbme 20: 27 mistakes. It was hard.
Nbme 23 (online): 246
Nbme 17: 7 mistakes
Nbme 19: 12 mistakes
Nbme 22(online): 263
Nbme 21: 16 mistakes
Nbme 16: 8 mistakes
UWSA 2: 269; Free 120: 89%. I did these two together to get exam feeling and it went pretty well.
Nbme 24 (online): 263
Nbme 18(online): 252
I read nbmes like the way we study uworld with FA. And that’s the way I revised my FA after uworld. It took me 3 to 4 days each nbme with new ones taking longer time. That’s how I ended up giving 3 months to assessments.
Exam day:
I reached prometric center at 8:00am and at 8:35 my exam was started. I don’t know how and why but I was super chill, had zero anxiety and was calm the whole time unexpectedly. I had 4 hours sleep. I took some chocolates and some biscuits with me; no energy drinks. I took 4-5 minutes break after each block. Marked 3-7 questions each block.
I found exam easier than uworld and nbmes. Uworld and nbmes prepare you for worst case scenario which is rare. Overall exam is doable.
Some suggestions from personal experience:
Every guy has a different approach and each approach has its own value. Whichever suits you just pick that and stick to it. Nobody is better tutor for you than you; you know yourself better. So whatever and whoever makes you understand and clears your concepts, go for it; it can be Kaplan notes and lectures, can be bnb etc. The main thing is to understand, now whoever does that, that doesn’t matter a lot. Whatever you choose is right for you but but but after analyzing everything; not just blindly decide something and going for it. No! The things which suits you, which makes you understand go for those things.
If you forget things then congratulations you are just as same as others. Because everyone forgets; it’s too volatile. It’s completely okay to forget. Don’t stressed up on this or else you won’t be fully functional. Just see what you forgot, remember it again. If you forget again, do the same. Eventually you will remember it and trust me this time you won’t forget for good.
Happy to answer any questions
Graduated from JSMU, YOG: Dec’17.
Preparation time: 21 Months
Uworld first pass, random: 83.7%
After wrong: 86%
I started my prep in 3rd year but I didn’t give much time to it due to extracurricular activities. I could not even finish Kaplan physiology in 2 months of vacations. Like this I spent my whole 2 years and graduated. The real deal started on January 8th, 2018. It took me 8 months to complete my FA and another 9 months to complete my Uworld. Yes, I am a slow reader. Step 1 has a lot of NBMEs. It took me almost 3 months to complete these assessments after Uworld, and then took my exam on October 9th, 2019.
Resources:
- I used Goljan (just general part), Kaplan(notes and lectures), 100 cases of ethics, Uworld Biostates review, bnb just for extremely selected topics, High yield anatomy(just for upper and lower limbs), pathoma, and of course FA and Uworld. And above all, GOOGLE :P
- Goljan was useful for me. It cleared a lot of concepts of mine. Could not do systems from it because it required a lot of time which I could not afford.
- I did Kaplan and FA side by side. And also noted down points on FA which I felt were important. Kaplan is way more useful if you are weak in basics.
- I did same with Pathoma.
- Started my Uworld on September 2018. Initially my scores were low but it got better with time. I am a slow reader so did less questions a day and no. of question I did varied each day. Noted down Uworld points on my FA so that I can revise it for later.
Assessments:
I took assessments in the following order:
Nbme 7 at 60% Uworld: 15 mistakes
Nbme 11 at 80% Uworld: 19 mistakes
Nbme 12 just after Uworld: 15 mistakes
UWSA 1: 269
Nbme 15: 13 mistakes
Nbme 20: 27 mistakes. It was hard.
Nbme 23 (online): 246
Nbme 17: 7 mistakes
Nbme 19: 12 mistakes
Nbme 22(online): 263
Nbme 21: 16 mistakes
Nbme 16: 8 mistakes
UWSA 2: 269; Free 120: 89%. I did these two together to get exam feeling and it went pretty well.
Nbme 24 (online): 263
Nbme 18(online): 252
I read nbmes like the way we study uworld with FA. And that’s the way I revised my FA after uworld. It took me 3 to 4 days each nbme with new ones taking longer time. That’s how I ended up giving 3 months to assessments.
Exam day:
I reached prometric center at 8:00am and at 8:35 my exam was started. I don’t know how and why but I was super chill, had zero anxiety and was calm the whole time unexpectedly. I had 4 hours sleep. I took some chocolates and some biscuits with me; no energy drinks. I took 4-5 minutes break after each block. Marked 3-7 questions each block.
I found exam easier than uworld and nbmes. Uworld and nbmes prepare you for worst case scenario which is rare. Overall exam is doable.
Some suggestions from personal experience:
Every guy has a different approach and each approach has its own value. Whichever suits you just pick that and stick to it. Nobody is better tutor for you than you; you know yourself better. So whatever and whoever makes you understand and clears your concepts, go for it; it can be Kaplan notes and lectures, can be bnb etc. The main thing is to understand, now whoever does that, that doesn’t matter a lot. Whatever you choose is right for you but but but after analyzing everything; not just blindly decide something and going for it. No! The things which suits you, which makes you understand go for those things.
If you forget things then congratulations you are just as same as others. Because everyone forgets; it’s too volatile. It’s completely okay to forget. Don’t stressed up on this or else you won’t be fully functional. Just see what you forgot, remember it again. If you forget again, do the same. Eventually you will remember it and trust me this time you won’t forget for good.
Happy to answer any questions
Images/noimage.png
Adim82
Characteristics Of Eschericia Coli,salmonella And Shigella And Their Relationship To Dysentery And Diarrhea
~3.9 mins read
INTRODUCTION
Bacteriology is the study of bacteria. There are few bacteria which cause either diarrhoea, dysentery or both. Diarrhoea can be defined as watery bowel movement 3 or more times in a day (WHO, 2019).Dysentery can be defined as an intestinal inflammation primarily of colon. It can cause mild or severe stomach cramp and severe diarrhoea with mucus or blood in the faeces (Alana Biggers,2017).Diarrhoea and dysentery are one of those medical conditions which are often used as synonym terms but they both have clear differences. The stool of diarrhoea is watery with or without cramps and pain but in dysentery there is mucoid stool that is presented with blood(Medimoon,2012).Example of bacteria that cause diarrhoea,dysentery or both include Escherichia coli,salmonella and shigella.
DISCUSSION
Escherichia coli(E.coli)
E.coli is a type of bacteria that normally live in the intestine. It is also found in the gut of some animals. Most of us associate E.coli with food poisoning but you can also get pneumonia and UTI too from different types of the bacteria. In fact, 75 to 95 percent of UTI are caused by E.coli.(Sabrina,2018)
Bacteriology is the study of bacteria. There are few bacteria which cause either diarrhoea, dysentery or both. Diarrhoea can be defined as watery bowel movement 3 or more times in a day (WHO, 2019).Dysentery can be defined as an intestinal inflammation primarily of colon. It can cause mild or severe stomach cramp and severe diarrhoea with mucus or blood in the faeces (Alana Biggers,2017).Diarrhoea and dysentery are one of those medical conditions which are often used as synonym terms but they both have clear differences. The stool of diarrhoea is watery with or without cramps and pain but in dysentery there is mucoid stool that is presented with blood(Medimoon,2012).Example of bacteria that cause diarrhoea,dysentery or both include Escherichia coli,salmonella and shigella.
DISCUSSION
Escherichia coli(E.coli)
E.coli is a type of bacteria that normally live in the intestine. It is also found in the gut of some animals. Most of us associate E.coli with food poisoning but you can also get pneumonia and UTI too from different types of the bacteria. In fact, 75 to 95 percent of UTI are caused by E.coli.(Sabrina,2018)
Characteristics of E.coli
E.coli is a gram negative bacteria in the family Enterobacteriaceae. It is rod shaped, non spore forming, motile with peritrichous flagella or nonmotile, and grows on macConkey agar (Wilson, 2001).They do not produce enterotoxin (drew, 2001).
E.coli is a gram negative bacteria in the family Enterobacteriaceae. It is rod shaped, non spore forming, motile with peritrichous flagella or nonmotile, and grows on macConkey agar (Wilson, 2001).They do not produce enterotoxin (drew, 2001).
Epidemiology
E.coli primarily cause diseases in neonates and young children, with most cases occurring in children6 months old(Nataro,2007).Disease can occur in adults if sufficiently high inoculants are ingested(Bopp 2007).Outbreaks have occurred in nurseries and day care centres and in adults that have consumed contaminated food from a buffet(Fields,2007).In developing countries, E.coli is highly prevalent and is an important cause of childhood diarrheal disease and dehydration associated deaths(Kaper,2007).Studies in Brazil, Mexico and South Africa have shown that 30 to 40 percent of infant diarrhoea can be attributed to E.coli(Strockbine,2007).
Mode Of Transmission
Contaminated food, water and fomites serve as vehicles for fecal/oral transmission of E.coli(Wilson,2001)
Clinical signs and symptoms of individuals infected with E.coli Nausea, vomiting, stomach cramps, diarrhoea that often is bloody ,fever of about 100 F to 101 F, Malaise, loss of appetite, mild dehydration(Sabrina,2018)
Clinical Diagnosis
The diagnosis of E.coli infection begins with an accurate history, physical exam and an analysis of a sample of stool from the patient(Sabrina,2018).A presumptive diagnosis is frequently made if the patient has symptoms of bloody diarrhoea. A definite diagnosis is based on culture of E.coli from the patient sample of stool on special culturing plates that are tested with antiserum that only react with E.coli (Sabrina, 2018).
How E.coli is identified in the lab Definitive identification is based on the isolation of the organism in the microbiology lab from clinical specimens such as blood urine, sputum or other fluids. Lumbar puncture and a CSF positive for E.coli establish its presence. E coli can be identified based on findings from serotyping, assay of adherence and DNA probes (Tarun, 2019).
Treatment and clinical management of patients with E.coli infection
Treatment with trimethoprim/sulfamethoxazole or quinolones reduce the duration of the diarrhoea(Baylis,2006)
Treatment of fluid and electrolyte loss is usually achieved through oral rehydration(Penn,2006).The use of WHO oral rehydration has been recommended(Thielman,2006).Intravenous rehydration may be necessary for infants, individuals with excessive vomiting ,or those with severe dehydration(guerrant,2006).Bismuth subsalicylate may decrease the amount of diarrhea and the duration of disease(Jenkins,2006).
Shigella
Shigella is a genus of bacteria that is Gram-negative. Its characteristics include that its facultative anaerobic, non spore forming, nonmotile and rod shaped. (Ryan,2004).
Epidemiology
It is common in developing countries where sanitation is bad.10 to 15 percent of enteric disease and 55 percent of bloody diarrhoea or dysentery of young children can be classified as shigellosis and the occurrence of these infections decreases significantly after five years of life(WHO,2019)
How Shigella is transmitted
It is transmitted by the fecal-oral route, including through direct person to person or sexual contact or indirectly through contaminated food, water or fomites.(CDC,2019)
E.coli primarily cause diseases in neonates and young children, with most cases occurring in children6 months old(Nataro,2007).Disease can occur in adults if sufficiently high inoculants are ingested(Bopp 2007).Outbreaks have occurred in nurseries and day care centres and in adults that have consumed contaminated food from a buffet(Fields,2007).In developing countries, E.coli is highly prevalent and is an important cause of childhood diarrheal disease and dehydration associated deaths(Kaper,2007).Studies in Brazil, Mexico and South Africa have shown that 30 to 40 percent of infant diarrhoea can be attributed to E.coli(Strockbine,2007).
Mode Of Transmission
Contaminated food, water and fomites serve as vehicles for fecal/oral transmission of E.coli(Wilson,2001)
Clinical signs and symptoms of individuals infected with E.coli Nausea, vomiting, stomach cramps, diarrhoea that often is bloody ,fever of about 100 F to 101 F, Malaise, loss of appetite, mild dehydration(Sabrina,2018)
Clinical Diagnosis
The diagnosis of E.coli infection begins with an accurate history, physical exam and an analysis of a sample of stool from the patient(Sabrina,2018).A presumptive diagnosis is frequently made if the patient has symptoms of bloody diarrhoea. A definite diagnosis is based on culture of E.coli from the patient sample of stool on special culturing plates that are tested with antiserum that only react with E.coli (Sabrina, 2018).
How E.coli is identified in the lab Definitive identification is based on the isolation of the organism in the microbiology lab from clinical specimens such as blood urine, sputum or other fluids. Lumbar puncture and a CSF positive for E.coli establish its presence. E coli can be identified based on findings from serotyping, assay of adherence and DNA probes (Tarun, 2019).
Treatment and clinical management of patients with E.coli infection
Treatment with trimethoprim/sulfamethoxazole or quinolones reduce the duration of the diarrhoea(Baylis,2006)
Treatment of fluid and electrolyte loss is usually achieved through oral rehydration(Penn,2006).The use of WHO oral rehydration has been recommended(Thielman,2006).Intravenous rehydration may be necessary for infants, individuals with excessive vomiting ,or those with severe dehydration(guerrant,2006).Bismuth subsalicylate may decrease the amount of diarrhea and the duration of disease(Jenkins,2006).
Shigella
Shigella is a genus of bacteria that is Gram-negative. Its characteristics include that its facultative anaerobic, non spore forming, nonmotile and rod shaped. (Ryan,2004).
Epidemiology
It is common in developing countries where sanitation is bad.10 to 15 percent of enteric disease and 55 percent of bloody diarrhoea or dysentery of young children can be classified as shigellosis and the occurrence of these infections decreases significantly after five years of life(WHO,2019)
How Shigella is transmitted
It is transmitted by the fecal-oral route, including through direct person to person or sexual contact or indirectly through contaminated food, water or fomites.(CDC,2019)
Clinical signs and symptoms
Frequent bouts of watery diarrhoea, abdominal cramping, nausea, vomiting, blood or mucus in stool(Erica,2016)
Clinical diagnosis
Confirming shigellosis involves taking a sample of your stool to be tested in a lab for the presence of shigella bacteria or their toxin(Mosby,2016)
How shigella is identified in the lab.
Shigella can be identified in the lab by total WBC count, stool examination, stool culture, enzyme immunoassay, polymerase chain reaction, fluorescent antibody test, enzyme linked DNA probes(Jaya,2018)
Treatment and clinical management
The clinician should rapidly assess the patients fluid and electrolyte status and institute parenteral or oral hydration along with antipyretics as needed. Nutritional supplementation of vitamin A can hasten clinical resolution in malnourished children. (Jaya,2018).
Frequent bouts of watery diarrhoea, abdominal cramping, nausea, vomiting, blood or mucus in stool(Erica,2016)
Clinical diagnosis
Confirming shigellosis involves taking a sample of your stool to be tested in a lab for the presence of shigella bacteria or their toxin(Mosby,2016)
How shigella is identified in the lab.
Shigella can be identified in the lab by total WBC count, stool examination, stool culture, enzyme immunoassay, polymerase chain reaction, fluorescent antibody test, enzyme linked DNA probes(Jaya,2018)
Treatment and clinical management
The clinician should rapidly assess the patients fluid and electrolyte status and institute parenteral or oral hydration along with antipyretics as needed. Nutritional supplementation of vitamin A can hasten clinical resolution in malnourished children. (Jaya,2018).
Conclusion
Shigella and E.coli are both causes of diarrhoea and dysentery and they both produce a potent shiga toxin during infection that cause severe and life threatening disease(Anita,2019)
Reference
1.Butler T,Speelman P,Kabir I. Et al. Colonic dysfunction during shigellosis Infect Dis. 1986;154:817(PubMed)
2.Tarun Madappa,2019,Escherichia coli infections workup 3.Bowen A(2016),chapter 3:infectious diseases related to travel 4.Levinson,warren E(2006).Review of medical microbiology and immunology(9th ed.)
Shigella and E.coli are both causes of diarrhoea and dysentery and they both produce a potent shiga toxin during infection that cause severe and life threatening disease(Anita,2019)
Reference
1.Butler T,Speelman P,Kabir I. Et al. Colonic dysfunction during shigellosis Infect Dis. 1986;154:817(PubMed)
2.Tarun Madappa,2019,Escherichia coli infections workup 3.Bowen A(2016),chapter 3:infectious diseases related to travel 4.Levinson,warren E(2006).Review of medical microbiology and immunology(9th ed.)
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