100+ Free FC Path(SA) Microbiol Practice Questions

Explanation: Pseudomonas aeruginosa is an oxidase-positive, obligate aerobic Gram-negative bacillus known for producing pyocyanin and pyoverdine pigments that impart a green colour on culture media. It is a major opportunistic pathogen in neutropenic patients. Escherichia coli and Klebsiella pneumoniae are oxidase-negative Enterobacterales, while Acinetobacter baumannii is oxidase-negative.

Explanation: Methicillin resistance in Staphylococcus aureus is mediated by the mecA or mecC gene carried on the staphylococcal cassette chromosome mec (SCCmec). This gene encodes penicillin-binding protein 2a (PBP2a), which has a low affinity for almost all beta-lactam antibiotics. TEM-1 beta-lactamase causes ampicillin resistance in Gram-negatives, NorA confers fluoroquinolone efflux, and ermA confers macrolide-lincosamide-streptogramin B (MLSB) resistance.

Explanation: Cryptococcus neoformans/gattii is the classic cause of fungal meningitis in severely immunocompromised patients, characterized by encapsulated yeasts visible on India ink preparation. The cryptococcal antigen lateral flow assay (LFA) performed on CSF or serum is highly sensitive and specific for definitive rapid diagnosis. Galactomannan tests for Aspergillus, beta-D-glucan is non-specific for invasive fungal infections but negative in Cryptococcus, and Histoplasma antigen tests for histoplasmosis.

Explanation: Crescentic or banana-shaped gametocytes are pathognomonic for Plasmodium falciparum, the predominant cause of severe malaria in South Africa. Plasmodium vivax, ovale, and malariae produce round or oval gametocytes. Early identification of P. falciparum is critical due to its potential for rapid progression to severe microvascular complication.

Explanation: Hand hygiene performed at the WHO '5 Moments' is universally recognized as the single most effective, evidence-based measure to interrupt pathogen transmission in healthcare settings. Environmental disinfection and UV light are adjunctive, while routine oral antibiotic decontamination promotes resistance and laminar airflow is restricted to specialized surgical operating theatres.

Explanation: Quantitative RT-qPCR targeting highly conserved regions such as the gag or pol gene is the gold standard for measuring HIV-1 RNA viral load in clinical plasma samples. Western blot and ELISA are antibody/antigen detection assays used primarily for diagnosis, while line probe assays are used for qualitative resistance genotyping.

Explanation: GeneXpert MTB/RIF Ultra is the cartridge-based real-time PCR assay recommended by WHO and the South African National Department of Health as the primary diagnostic test for pulmonary TB and simultaneous detection of rpoB mutations conferring rifampicin resistance. QuantiFERON tests for latent infection, MGIT is liquid culture, and MTBDRsl is used for second-line drug resistance testing.

Explanation: Streptococcus pneumoniae is classically identified by alpha-hemolysis on blood agar, sensitivity to optochin (ethylhydrocupreine hydrochloride), and solubility in bile salts (sodium desoxycholate). Streptococcus pyogenes and agalactiae are beta-hemolytic, while Enterococcus faecalis is optochin-resistant and bile-esculin positive.

Explanation: Under EUCAST definitions updated since 2019, 'I' stands for 'Susceptible, Increased exposure'. It indicates that there is a high probability of therapeutic success if exposure to the agent is increased by adjusting the dosage regimen or concentration at the infection site. It replaces the old ambiguous term 'Intermediate'.

Explanation: Low IgG avidity indicates weak binding affinity characteristic of antibodies produced during primary infection acquired within the preceding 3 to 4 months. High IgG avidity rules out recent primary infection, confirming past immunity or distant acquisition. This distinction is critical in pregnancy to assess congenital transmission risk.