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Human Health and Disease Set-2

Practice Important MCQs for Human Health and Disease — Biology, Class 12.

This chapter is crucial for both board and competitive exams because it links everyday diseases to core biological concepts like immunity, transmission dynamics, herd immunity, diagnostics, and antibiotic/vaccine-driven evolution. Questions commonly test quantitative reasoning (e.g., $R_0$ , PPV), application of immunology (IgA vs IgG, vaccine types), and interpretation of real-world outbreaks and resistance patterns.

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Q1. A viral pathogen has a basic reproduction number $R_0=3$ . Using the herd-immunity threshold formula $H = 1 - \dfrac{1}{R_0}$ , what minimum percentage of the population must be immunized to prevent sustained transmission?

(A)

50%

(B)

33.3%

(C)

66.7%

(D)

75%

Q2. In a town of 100,000 people there are 400 new cases of a disease each year and the average duration of active disease is 3 years. Using the relation $P = I \times D$ (where $P$ is prevalence per person, $I$ is incidence per person-year, and $D$ is duration in years), what is the point prevalence expressed per 1000 population?

(A)

1.2 per 1000

(B)

12 per 1000

(C)

48 per 1000

(D)

4 per 1000

Q3. Two cholera vaccine formulations are compared: Vaccine L is an oral live-attenuated strain that transiently replicates in the gut; Vaccine K is an intramuscular killed whole-cell vaccine. Which vaccination strategy is most likely to induce stronger secretory IgA-mediated protection at the intestinal mucosa and why?

(A)

Vaccine K — intramuscular killed vaccines mainly induce systemic IgG and only limited secretory IgA at intestinal mucosa

(B)

Both L and K produce equivalent mucosal IgA because antigen composition is the same

(C)

Vaccine K — even with adjuvant, intramuscular delivery is less effective at generating mucosal (secretory) IgA than mucosal replication of live vaccine

(D)

Vaccine L — replication at the mucosal surface engages local inductive sites (e.g., Peyer’s patches) and preferentially elicits secretory IgA

Q4. A diagnostic test has sensitivity $Se=98\%$ and specificity $Sp=95\%$ . In a population where disease prevalence $Prev=1\%$ , calculate the positive predictive value using

PPV=\dfrac{Se\times Prev}{Se\times Prev + (1-Sp)\times(1-Prev)}.

Approximately what percentage of positive test results are true positives?

(A)

≈ 16.5%

(B)

≈ 66%

(C)

≈ 95%

(D)

≈ 49%

Q5. Assertion: Widespread use of a broad‑spectrum antibiotic in a community can increase the prevalence of resistance to an unrelated antibiotic that is not used in that community.
Reason: Broad‑spectrum antibiotics induce new chromosomal mutations that create resistance to unrelated antibiotics, and those mutations then spread.

(A)

Both Assertion and Reason are true and the Reason correctly explains the Assertion

(B)

Both Assertion and Reason are true but the Reason does not correctly explain the Assertion

(C)

Assertion is true but the Reason is false

(D)

Assertion is false but the Reason is true

Q6. In a homogeneous population the basic reproduction number for a contagious disease is $R_0=5$ . Using the herd immunity threshold formula $H = 1 - \dfrac{1}{R_0}$ , what minimum fraction of the population must be immunized with a perfectly effective vaccine to prevent sustained transmission?

(A)

$60\%$

(B)

$80\%$

(C)

$50\%$

(D)

$20\%$

Q7. A diagnostic assay has sensitivity $Se=0.90$ and specificity $Sp=0.95$ . Using Bayes' formula for positive predictive value

PPV=\dfrac{Se\times P}{Se\times P + (1-Sp)\times(1-P)},

calculate the approximate $PPV$ when disease prevalence $P=0.05$ .

(A)

$90\%$

(B)

$95\%$

(C)

$5\%$

(D)

$48.6\%$

Q8. For a disease with $R_0=3$ , let the fraction with prior natural immunity be $i=0.10$ , vaccine coverage $v=0.60$ and vaccine efficacy $e=0.80$ . If the susceptible fraction is $S=1 - i - v e$ and the effective reproduction number is $R_e = R_0 S$ , what is the estimated $R_e$ ?

(A)

$R_e = 1.26$

(B)

$R_e = 1.00$

(C)

$R_e = 0.42$

(D)

$R_e = 2.40$

Q9. A hospital stopped using antibiotic X and initially X-resistant isolates fell, but after 12 months resistance returned to previous levels despite no clinical use of X. Which explanation most plausibly accounts for both persistence and rebound of resistance?

(A)

Resistant strains simply outcompeted susceptibles because resistance always confers a fitness advantage even without antibiotic.

(B)

Recolonization from outside sources alone explains the rebound; resistance genes must have been completely lost inside the hospital.

(C)

A combination of factors: compensatory mutations reduced the fitness cost of resistance, resistance genes on plasmids were co-selected by other agents (other antibiotics/heavy metals), and horizontal transfer to commensals created a reservoir that reintroduced resistance.

(D)

Surveillance error (false laboratory reports) is the most likely single cause of the observed pattern.

Q10. A vaccine reduces disease severity and mortality but does not prevent infection or onward transmission (a non-sterilizing vaccine). Several years after mass vaccination, more virulent strains of the pathogen become common. Which explanation best accounts for this evolutionary outcome?

(A)

Herd immunity eliminated low-virulence strains, leaving only highly virulent strains by chance.

(B)

Because vaccinated hosts survive infection but still transmit, high-virulence strains that formerly killed hosts too quickly can spread and be selected, so non-sterilizing vaccination favors evolution of greater virulence.

(C)

Vaccine-induced immediate neutralization of the virus selects specifically for low-virulence strains to become dominant.

(D)

This pattern can only be due to antibody-dependent enhancement (ADE) caused by the vaccine.

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Human Health and Disease Set-2 MCQ Online Practice Test | Class 12 | Quiz Tweek

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