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Human Health and Disease Set-6 MCQ Online Practice Test | Class 12 | Quiz Tweek

Human Health and Disease Set-6

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

“Human Health and Disease” is a high-yield chapter because it connects core CBSE biology concepts (immunity, infection, vaccines, and disease prevention) with exam-relevant reasoning used in NEET/JEE-style questions (thresholds, test accuracy, and mechanisms of host-pathogen interactions). Mastery of these ideas helps you predict outcomes like herd immunity, vaccine effectiveness, and immune-system failure patterns—frequently asked both directly and through data-based problems.

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Q1. A pathogen has basic reproduction number $R_0 = 5$ . Using the herd-immunity threshold $p_c = 1 - \dfrac{1}{R_0}$ , what minimum fraction of the population must be immune (by vaccination or prior infection) to prevent sustained transmission?

(A)

60%

(B)

75%

(C)

80%

(D)

50%

Q2. A vaccine has efficacy $E = 0.8$ (80%) and the disease has $R_0 = 3$ . Using the relation $p = \dfrac{1 - 1/R_0}{E}$ for required vaccination coverage to achieve herd immunity, what minimum percentage of the population must be vaccinated? (Round to one decimal place.)

(A)

83.3%

(B)

66.7%

(C)

75.0%

(D)

90.0%

Q3. An infection is caused by an obligate intracellular bacterium. Which class of antibiotics is least likely to be effective because it poorly penetrates eukaryotic host cells?

(A)

Tetracyclines (e.g., doxycycline)

(B)

Macrolides (e.g., azithromycin)

(C)

Fluoroquinolones (e.g., ciprofloxacin)

(D)

Aminoglycosides (e.g., gentamicin)

Q4. Patients with chronic granulomatous disease (CGD) have defective NADPH oxidase and suffer recurrent infections, particularly by catalase-positive organisms such as Staphylococcus aureus and Serratia marcescens. Which explanation best accounts for the selective susceptibility to catalase-positive bacteria?

(A)

Defective NADPH oxidase prevents superoxide formation so myeloperoxidase (MPO)-mediated conversion of $H_2O_2$ to $HOCl$ cannot occur, rendering killing ineffective.

(B)

Catalase-positive bacteria degrade $H_2O_2$ produced by microbes; since phagocytes with defective NADPH oxidase cannot produce their own reactive oxygen species, they rely on microbe-derived $H_2O_2$ , which is destroyed by catalase, allowing these bacteria to survive.

(C)

A specific deficiency of complement components $C_5$ – $C_9$ in CGD prevents membrane attack complex formation that is essential to kill catalase-positive bacteria.

(D)

CGD patients cannot produce opsonizing antibodies to polysaccharide capsules, and most catalase-positive bacteria evade phagocytosis due to uncapsulated surface antigens.

Q5. In areas with low polio immunization coverage, outbreaks of vaccine-derived poliovirus (VDPV) have occurred after use of oral polio vaccine (OPV). Which explanation best accounts for this phenomenon?

(A)

The live attenuated strains in OPV replicate in the gut, may accumulate mutations that restore neurovirulence, and then circulate among unimmunized individuals lacking immunity, causing paralytic disease.

(B)

Low coverage reduces maternal antibodies so vaccinated infants cannot neutralize OPV and therefore the vaccine strain directly causes disease by retaining original virulence.

(C)

OPV viruses acquire neurovirulence mainly by recombining with other enteroviruses in the gut, and this can occur even when most people are immune.

(D)

Breakdown of the vaccine cold chain increases the mutation rate in OPV strains, causing reversion to a virulent phenotype in poorly immunized communities.

Q6. A communicable disease has basic reproduction number $R_0=4$ . A vaccine with efficacy $E=75\%$ ( $0.75$ ) is available. Using the herd-immunity threshold $H=1-\dfrac{1}{R_0}$ , what minimum fraction of the population must be vaccinated to achieve herd immunity (assume homogeneous mixing and no prior immunity)?

(A)

$75\%$

(B)

$50\%$

(C)

$80\%$

(D)

$100\%$

Q7. The Mantoux test for tuberculosis has sensitivity $Se=90\%$ and specificity $Sp=85\%$ . In a screened population the prevalence of active TB is $P=5\%$ . What is the approximate Positive Predictive Value (PPV) of the test? (You may use $PPV=\dfrac{Se\times P}{Se\times P + (1-Sp)\times(1-P)}$ .)

(A)

$50\%$

(B)

$24\%$

(C)

$10\%$

(D)

$76\%$

Q8. A bacterial infection harbors $10^{9}$ cells. The spontaneous mutation rate to resistance against drug A is $10^{-7}$ per cell generation and independently $10^{-7}$ for drug B. If resistance to both drugs requires independent mutations in two different genes, estimate the expected number of bacteria already resistant to both drugs before treatment.

(A)

Approximately $10^{-5}$ bacteria (virtually none)

(B)

About $10^{2}$ bacteria

(C)

Around $10^{4}$ bacteria

(D)

Nearly $10^{9}$ bacteria (most of the population)

Q9. A newborn has maternal IgG level of 80 units at birth which decays with half-life $t_{1/2}=30$ days. A live attenuated vaccine will be neutralized if maternal IgG exceeds 5 units. Using exponential decay $N=N_0\left(\tfrac{1}{2}\right)^{t/t_{1/2}}$ , after approximately how many months should the infant be vaccinated so that maternal IgG falls below 5 units? (Round to the nearest whole month.)

(A)

4 months

(B)

3 months

(C)

5 months

(D)

9 months

Q10. In a community $90\%$ of people are vaccinated with a measles vaccine of efficacy $E=95\%$ . For measles with $R_0=15$ , calculate the effective reproduction number $R_e=R_0(1-pE)$ and state whether a single introduced case is likely to cause an outbreak.

(A)

$R_e\approx 0.2$ — disease will die out ( $R_e<1$ )

(B)

$R_e\approx 2.2$ — outbreak likely ( $R_e>1$ )

(C)

$R_e\approx 15$ — vaccination has no effect

(D)

$R_e\approx 1.0$ — at epidemic threshold

Q11. A communicable disease has basic reproduction number $R_0 = 3$ . Using the herd-immunity threshold formula $1 - \frac{1}{R_0}$ , the minimum percentage of the population that must be immune to prevent sustained transmission is:

(A)

$50\%$

(B)

$66.7\%$

(C)

$33.3\%$

(D)

$75\%$

Q12. In an outbreak investigation 1000 vaccinated people were followed and 20 developed the disease, while 500 unvaccinated people were followed and 50 developed the disease. Using $VE = \dfrac{AR_U - AR_V}{AR_U}\times100\%$ (where $AR_V$ and $AR_U$ are attack rates in vaccinated and unvaccinated groups), the vaccine efficacy is closest to:

(A)

$70\%$

(B)

$90\%$

(C)

$50\%$

(D)

$80\%$

Q13. A vaccine is being designed against an intracellular respiratory bacterium; the primary goals are (i) induction of strong CD8+ cytotoxic T‑cell responses (via antigen presentation on MHC‑I) and (ii) robust mucosal immunity in the respiratory tract. Which vaccine candidate is most likely to meet both goals?

(A)

Live‑attenuated vaccine administered intranasally

(B)

Killed (inactivated) whole‑cell vaccine given intramuscularly with an alum adjuvant

(C)

Purified subunit vaccine (protein antigen) given intramuscularly with alum

(D)

Toxoid vaccine given orally

Q14. A screening test has sensitivity $95\%$ and specificity $90\%$ . Disease prevalence in the screened population is $1\%$ . Using the positive predictive value formula

PPV = \frac{\text{sensitivity}\times\text{prevalence}}{\text{sensitivity}\times\text{prevalence} + (1-\text{specificity})\times(1-\text{prevalence})},

the probability that a person who tests positive actually has the disease is approximately:

(A)

$51.1\%$

(B)

$9.0\%$

(C)

$8.8\%$

(D)

$95\%$

Q15. For a disease with basic reproduction number $R_0 = 4$ , a vaccine with efficacy $E = 80\%$ (i.e. $E=0.8$ ) is available. The critical vaccination coverage $p_c$ required to interrupt transmission when vaccine efficacy is imperfect is given by

p_c = \frac{1 - 1/R_0}{E}.

The minimum percentage of the population that must be vaccinated is closest to:

(A)

$75\%$

(B)

$93.75\%$

(C)

$37.5\%$

(D)

$56.25\%$

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Human Health and Disease Set-6

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