Question:

There are many explanatory sources, such as pictures, videos, and audio clips to explain these explanations and questions and explain the answers, but you must subscribe first so that you can enjoy all these advantages. We have many subscription plans at the lowest prices. Don't miss today's offer. Subscribe

The graph below shows the yearly incidence of viral hepatitis per 100,000 population from 1982-2012 in the United States (US).

Show Explanatory Sources

During this period, the total population in the US continued to increase, as shown in the table below:

Year	1982	1987	1992	1997	2002	2007	2012
US population (in millions)	~230	~243	~258	~275	~290	~303	~315

At any given time point, the number of individuals at risk for hepatitis infection is assumed to be equal to the total US population. Based on these data, which of the following is most likely to be true?

After 2006, hepatitis B prevalence surpassed hepatitis A and hepatitis C prevalence

In 1987, there were as many individuals with hepatitis A as with hepatitis B in the US

In 1989, there were more individuals with hepatitis A than with hepatitis B in the US

In 1997, there were more new cases of hepatitis A than of hepatitis B and C combined

The number of new cases of hepatitis A diagnosed in 1982 and 1998 were exactly the same

Welcome to USMLEPREPS!

Master your USMLE exams with our comprehensive question bank.

Subscribe to our Life Time Plan for unlimited access to Step 1, Step 2, and Step 3 preparation materials.

Don't miss this offer!

Hurry up!

: : Get The Offer

Unlimited Access Step ( one, two and three ).

Priority Access To New Features.

Free Lifetime Updates Facility.

Dedicated Support.

Explanation:

Incidence corresponds to the number of new cases of a disease diagnosed in a population at risk over a given time period. In 1997, incidence was >10 new cases/100,000 population for hepatitis A virus (HAV), <5/100,000 for hepatitis B virus (HBV), and <3/100,000 for hepatitis C virus (HCV). Assuming, as instructed, that the entire United States (US) population is at risk (reasonable theory given low number of hepatitis cases among the total population), then, in 1997 (US population ~275 million):

New HAV cases in 1997 = >10 new cases/100,000 population × 275,000,000 population = >27,500
New HBV cases in 1997 = <5/100,000 × 275,000,000 = <13,750
New HCV cases in 1997 = <3/100,000 × 275,000,000 = <8,250

There were thousands more new HAV (>27,500) than HBV and HCV cases combined (<13,750 + <8,250 = <22,000). The calculations incorporating total population (included for completion) are not needed to answer the question.

After 2006, HBV incidence surpassed HAV and HCV incidence. However, no information can be deduced regarding prevalence, which is the total number of cases in the population (Choice A). Compared to new HBV cases, there were approximately just as many new HAV cases in 1987 and more new HAV cases in 1989. However, the total number of HAV or HBV cases in the population cannot be calculated without additional information. In fact, there were likely more total cases (ie, higher prevalence) of HBV as it can progress to a chronic form (Choices B and C).

A simplified diagram compares incidence and prevalence to adding new drops (incident cases) into a water-containing sink (prevalent cases).

(Choice E) HAV incidence per 100,000 population was similar in 1982 and 1998. However, the overall US population, and consequently the number of new HAV cases, was different. Assuming HAV incidence was ~10 new cases/100,000 population for both years, then, in 1982 (US population ~230 million):

New HAV cases in 1982 = 10/100,000 × 230,000,000 = 23,000

Also, in 1998 (US population ~275 million at least):

New HAV cases in 1998 = 10/100,000 × 275,000,000 = 27,500

Even with different incidences, it is unlikely that exactly the same number of new HAV diagnoses were made.

Educational objective:
Incidence corresponds to the number of new cases of a disease in a certain population at risk over a given time period. Prevalence is the total number of cases in the population over a given period.