# DBT JRF 2020 — Question 54

Exam: DBT JRF 2020 · Section: B · Topic: Microbiology · Difficulty: Easy

## Question

Which of the following represents the correct sequence of steps in pathogenesis?

## Options

- **A.** adhesion, infection, exposure, invasion
- **B.** invasion, infection, adhesion, exposure
- **C.** adhesion, exposure, infection, invasion
- **D.** exposure, adhesion, invasion, infection

## Answer

Option D

## Explanation

Gel filtration (size exclusion chromatography, SEC): large molecules are excluded from pores → elute first (low elution volume); small molecules enter pores → elute last (high elution volume). $\log(M_r)$ is linearly proportional to elution volume: $\log(M_r) = a - b \times V_e$ . Known data points: $(220 \text{ mL}, \log(10^5) = 5)$ and $(300 \text{ mL}, \log(10^4) = 4)$ . Slope: $b = \dfrac{5-4}{300-220} = \dfrac{1}{80}$ . Intercept: $a = 5 + \dfrac{220}{80} = 5 + 2.75 = 7.75$ . At $V_e = 140 \text{ mL}$ (extrapolated beyond the largest protein): $\log(M_r) = 7.75 - \dfrac{1}{80} \times 140 = 7.75 - 1.75 = 6.0$ . $M_r = 10^6 = 1.0 \times 10^6 \text{ Da}$ .

Source: https://mypyqbuddy.com/questions/which-of-the-following-represents-the-correct-sequence-6a05a8a02adafec47029324e