(a) (i) Define inflammation. Describe the cardinal features of acute inflammation. (10 marks) (ii) Describe the pathogenesis of…

(a)(i) Inflammation and the cardinal features of acute inflammation (10 marks)

Definition. Inflammation is the local response of living, vascularised tissue to injury (infection, trauma, chemical or immunological damage). It brings plasma proteins and leucocytes to the site to neutralise and remove the injurious agent, and prepares the tissue for repair.

Cardinal features. Celsus described four signs (rubor, calor, tumor, dolor), and Virchow added the fifth (functio laesa). Each is explained by vascular and cellular events that follow injury.

• Rubor (redness): transient arteriolar constriction is followed by vasodilatation and opening of capillary beds under histamine, nitric oxide and prostaglandins. The increased blood flow (active hyperaemia) makes the area red.
• Calor (heat): the same increase in blood flow brings warm blood to the surface, and in a large lesion the pyrogens (IL-1, TNF, PGE₂) add systemic fever.
• Tumor (swelling): increased vascular permeability (endothelial contraction by histamine, bradykinin and leukotrienes, or direct endothelial injury) lets protein-rich fluid escape, the exudate. The high oncotic pressure of the tissue fluid and the cell infiltrate add to the swelling.
• Dolor (pain): bradykinin, prostaglandin E₂ and serotonin stimulate the pain endings, and the pressure of the exudate on the nerve endings adds to it.
• Functio laesa (loss of function): the result of pain, swelling and tissue damage, which limit movement of the part.

Cellular events. Blood flow slows (stasis) as fluid leaves, and leucocytes move to the vessel wall (margination). They roll (selectins), adhere firmly (integrins binding ICAM-1) and cross the wall (diapedesis, using PECAM-1). Under chemotactic signals (C5a, leukotriene B₄, IL-8, bacterial products) they migrate to the site, where they phagocytose the agent and kill it by oxygen-dependent and oxygen-independent mechanisms. Neutrophils dominate for the first 24 hours, and macrophages follow.

(a)(ii) Secondary tuberculosis and TB in AIDS (5 + 5 = 10 marks)

Pathogenesis of secondary (post-primary) tuberculosis. Secondary TB arises in a host already sensitised by primary infection. It follows either the reactivation of dormant bacilli from the primary complex (or another focus), or a fresh exogenous reinfection, usually when immunity is lowered (malnutrition, diabetes, HIV, steroids, alcohol, old age).

• The lesion starts in the apex of the upper lobe (Simon's focus, or the Assmann focus in the infraclavicular region) because the oxygen tension is high there, the organism is strictly aerobic, and the lymph drainage and perfusion are poorer.
• The hypersensitivity (type IV) and acquired immunity shape the lesion. There is a brisk reaction with caseous necrosis, which is contained, so the lesion stays localised, the hilar nodes are spared or only mildly involved, and fibrosis and calcification result.
• Liquefaction of the caseous material and its discharge through a bronchus leaves a cavity, which spreads bacilli by the bronchi (bronchogenic spread) and gives the patient an infectious sputum. Erosion of a vessel causes haemoptysis. Bloodborne spread produces miliary TB, and swallowed sputum causes intestinal disease.
• The lesion may heal by fibrosis and calcification, or progress as fibrocaseous, cavitary disease.

Microscopic features of tuberculosis in AIDS (five). They depend on the CD4 count. In early HIV, the granulomas are typical, but with advanced immunosuppression:

• Poorly formed or absent granulomas, with few epithelioid cells.
• Few Langhans giant cells, and few lymphocytes (loss of CD4 helper cells).
• Extensive necrosis, often not the typical caseation, and sometimes a suppurative (neutrophil-rich) necrosis.
• Numerous acid-fast bacilli in the necrotic areas and inside macrophages on Ziehl–Neelsen stain.
• Sheets of foamy macrophages full of bacilli, with a diffuse infiltrate in place of nodules (and infection with atypical mycobacteria such as M. avium complex in the same pattern).

(b)(i) Mechanisms of antimicrobial resistance (5 marks)

Biochemical mechanisms.

• Enzymatic inactivation of the drug: β-lactamases (penicillin resistance in staphylococci, extended-spectrum β-lactamases and carbapenemases such as NDM-1 and KPC in Gram-negative bacilli), aminoglycoside-modifying enzymes, and chloramphenicol acetyltransferase.
• Altered target site: PBP2a in MRSA (mecA gene), the D-Ala-D-Lac terminus in vancomycin-resistant enterococci, altered DNA gyrase (fluoroquinolones), methylated ribosomal RNA (erm gene, macrolides), and altered RNA polymerase (rifampicin).
• Reduced permeability: loss of outer-membrane porins, as in carbapenem-resistant Pseudomonas.
• Active efflux: pumps expel the drug, as with tetracycline and fluoroquinolones (NorA) and the multidrug MexAB-OprM pump of Pseudomonas.
• Metabolic bypass: altered dihydropteroate synthase or a resistant dihydrofolate reductase (sulphonamides and trimethoprim), and biofilm formation.

Genetic basis. Resistance may be intrinsic, or acquired by chromosomal mutation (for example rifampicin resistance, which is selected by drug pressure), or by horizontal gene transfer. Horizontal transfer is by conjugation (R plasmids, transposons, integrons carrying several genes), transduction (by bacteriophage) and transformation (uptake of naked DNA). Misuse and overuse of antibiotics select the resistant strains.

(b)(ii) Third-generation cephalosporins (5 marks)

Names.

• Parenteral: cefotaxime, ceftriaxone, ceftazidime, cefoperazone and ceftizoxime.
• Oral: cefixime, cefpodoxime proxetil, cefdinir, cefditoren pivoxil and ceftibuten.

Therapeutic indications. Meningitis (ceftriaxone, cefotaxime), gonorrhoea (single dose of ceftriaxone), typhoid (enteric) fever, severe sepsis and Gram-negative bacteraemia, community-acquired pneumonia, complicated urinary tract infection, spontaneous bacterial peritonitis, Lyme disease and surgical prophylaxis. Ceftazidime and cefoperazone are used against Pseudomonas aeruginosa, including in neutropenic fever.

Side effects. Hypersensitivity (cross-reaction with penicillin in about 5–10%), diarrhoea and superinfection (Clostridioides difficile, candidiasis), thrombophlebitis, biliary sludge and pseudolithiasis (ceftriaxone), hypoprothrombinaemia with bleeding and a disulfiram-like reaction with alcohol (cefoperazone), neutropenia and haemolytic anaemia, and raised liver enzymes. Ceftriaxone is contraindicated in neonates who need calcium-containing infusions (precipitation) and in hyperbilirubinaemic neonates.

(c)(i) Candida (5 + 3 + 2 = 10 marks)

1. Five clinically important species (5 marks). Candida albicans (the commonest), C. tropicalis, C. glabrata, C. parapsilosis and C. krusei. C. auris is an emerging, multidrug-resistant species, and C. dubliniensis is closely related to C. albicans.

2. Laboratory diagnosis (3 marks).

• Specimens: swabs and scrapings, sputum, urine, blood, CSF, tissue and catheter tips.
• Microscopy: KOH mount or Gram stain shows oval budding yeast cells (3–6 µm) with pseudohyphae. Tissue is stained with PAS or silver.
• Culture: Sabouraud dextrose agar at 25–37 °C gives creamy white, pasty colonies. Chromogenic agar (CHROMagar) separates species by colour (C. albicans green). Blood culture is used for candidaemia.
• Identification: the germ tube test (positive for C. albicans in serum at 37 °C in 2–3 hours), chlamydospore formation on corn meal agar, sugar assimilation and fermentation tests, MALDI-TOF and PCR.

3. Seromarker of invasive candidiasis (2 marks). Serum (1→3)-β-D-glucan, a cell wall component, is the marker (with mannan antigen and anti-mannan antibody as an alternative). It is not specific for Candida.

(c)(ii) Aspergillus infection (10 marks)

Clinical manifestations (any five).

• Allergic bronchopulmonary aspergillosis (ABPA): a type I and type III hypersensitivity in asthmatics and cystic fibrosis, with wheeze, eosinophilia, raised IgE and fleeting infiltrates.
• Aspergilloma (fungal ball): saprophytic growth in a pre-existing lung cavity (old tuberculosis), causing haemoptysis.
• Invasive pulmonary aspergillosis: in neutropenic and transplant patients, with fever, pleuritic pain, haemoptysis and angioinvasion (halo sign on CT), and dissemination to the brain.
• Chronic pulmonary aspergillosis: chronic cavitary and necrotising disease in a lung with structural damage.
• Extrapulmonary: allergic or invasive fungal sinusitis, otomycosis, fungal keratitis, cutaneous infection and endocarditis. Aspergillus flavus also causes aflatoxin poisoning and liver cancer.

Specific antigen test. The galactomannan assay, an ELISA for the cell wall polysaccharide galactomannan in serum or bronchoalveolar lavage fluid (positive at an index of 0.5 or more in serum), is used for invasive aspergillosis. False positives occur with some antibiotics such as piperacillin–tazobactam.