A-LevelBiologyInfectious diseases9700/22 · May/June 2025 · Q412 Marks

Tuberculosis (TB) is an infectious disease that is caused by a bacterial pathogen. The pathogen has mechanisms to avoid digestion by phagocytes. Macrophages may engulf the bacteria, but in some cases the bacteria remain alive within the cells instead of being killed.

(a)Although there are two main species of bacterium causing TB, it is rare for the species that causes bovine TB in cattle to infect humans. Name the species of bacterium that is the main cause of TB in humans.[1]
(b)Explain how the pathogen named in (a) is transmitted from a person with the disease to a person who is uninfected.[2]
(c)State the type of therapeutic drug used to treat TB.[1]
(d)Fig. 4.1 is a photomicrograph of a section of lung tissue taken from a person who has not been infected with the bacterial pathogen and who does not have TB. [Figure 4.1] Blood vessels and some structures of the gas exchange system are visible in Fig. 4.1. On Fig. 4.1, use a label line and label: • a bronchus • a bronchiole • a blood vessel.[3]
(e)In some people with TB, areas known as granulomas may form in lung tissue as part of an immune response to the pathogen. Fig. 4.2 is a photomicrograph of a granuloma in lung tissue. [Figure 4.2] With reference to Fig. 4.1 and Fig. 4.2, describe and explain how the changes that occur as a result of granuloma formation: • can affect gas exchange and harm the health of an infected person • may help to prevent TB developing in other parts of the body.[5]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2025). The mark or mark type is shown in brackets.

[1] Mycobacterium tuberculosis ; must be spelled correctly
[1] (infected), person, coughs / breathes (out) / talks / sneezes / AW, and (uninfected) person, inhales / inspires / breathes in
[1] aerosol infection / aerosol transmission / droplet infection / droplet transmission must be a term OR organism / pathogen / bacteria / M. tuberculosis, in, airborne droplets / droplets in air
[1] antibiotic
[1] label line and label to: bronchus
[1] label line and label to: bronchiole
[1] label line and label to: blood vessel
[1] (Gas exchange & health) fewer, bronchioles / alveoli / alveolar capillaries OR granuloma / described, causes, damage / lesions / airway lumen to decrease / AW ; A takes up lung space I blocks airways
[1] (Gas exchange & health) consequence ; e.g. less air enters alveoli / reduced surface area (for gas exchange) context is fewer alveoli (fewer capillaries) fewer red blood cells / less haemoglobin, to take up oxygen (granuloma) causes (partial) air blockage / prevents alveolar function / AW
[1] (Gas exchange & health) less oxygen, reaches tissues / to the (rest of) body (in circulation)
[1] (Gas exchange & health) consequence ; e.g. tiredness / fatigue / less mobile increases blood pressure / strain on heart rapid breathing / causes breathlessness A difficulty breathing I affects breathing
[1] (Prevent spread) infected (lung) cells / pathogen, prevented from entering, circulation / bloodstream
[1] (Prevent spread) granuloma acts to, confine / localise / isolate, pathogen ; A description of cells within granuloma acting as the barrier
[1] (Prevent spread) many / surrounded by / AW, cells of the immune system / named ; I white blood cells e.g. lymphocytes / plasma cells / phagocytes / macrophages / neutrophils / multinucleated giant cells note: many macrophages in a giant cell is not mp7
[1] (Prevent spread) other detail of immune response stimulated ; e.g. antigen presentation antibody formation (against M. tuberculosis) phagocytosis of, infected cells / pathogen cytokine release
[1] (Prevent spread) AVP ; e.g multinucleated giant cells, more effective as a large cell / increase chance of detecting pathogen result of immune response prevents, population increase / bacteria reproducing /AW another detail of immune response