MLT GlobeHistopathology: Fixation & fixatives
50 Histopathology MCQs on Fixation & fixatives with answers and explanations.
Which acid, found in some fixatives, is only partly ionised in water?
Acetic acid (CH3COOH) is a weak acid; HCl, H2SO4 and HNO3 dissociate completely and are strong acids.
A skin biopsy for routine H&E sections should be sent to the laboratory in:
Routine biopsies go in about ten times their volume of 10% NBF; saline or ice does not fix, and weak alcohol is inadequate.
An endometrial biopsy is sent to exclude tuberculous endometritis by culture. It should be placed in:
Culture needs viable mycobacteria; fixatives such as formalin or alcohol kill them, so a portion goes fresh in sterile saline.
Which fixative is used most widely in routine histopathology laboratories?
Buffered formalin is cheap, penetrates well and preserves morphology and antigens; the others are used mainly in special mixtures.
Ethanol fixes tissue by which mechanism?
Alcohols are coagulant fixatives that remove water and precipitate proteins; methylene bridges are formed by formaldehyde.
Immediately after removal from the patient, a routine tissue specimen should go into:
Prompt fixation stops autolysis and bacterial decay; clearing and dehydration are later processing steps.
Tissue post-fixed in osmium tetroxide shows black deposits. Which component do they mark?
Osmium tetroxide reacts with unsaturated lipids and is reduced to black osmium, so it both fixes and stains fat.
Which is NOT a desirable property of a good fixative?
A fixative should preserve size and structure; shrinkage is an artefact, not an aim.
A routine small biopsy in 10% NBF generally needs about how long for adequate fixation?
Formalin penetrates quickly but cross-linking takes hours; too short gives poor morphology, too long masks antigens.
Which fixative is standard for specimens loaded on routine automated tissue processors?
NBF is compatible with routine processing, reagent recycling and IHC; picric and mercury fixatives need special handling.
For adequate fixation, the volume of fixative should be at least how many times the tissue volume?
A fixative-to-tissue ratio of at least 10:1 (ideally 15–20:1) is recommended so the fixative is not depleted or diluted. Smaller volumes lead to poor, uneven fixation.
Bouin's fluid gives tissue a yellow colour that must be washed out. This colour comes from:
Bouin's contains picric acid, formalin and acetic acid; the picric acid stains tissue yellow. Excess is removed with 70% alcohol before processing.
Which step of routine tissue preparation stops autolysis and preserves structure?
Fixation cross-links or precipitates proteins, halting enzymatic breakdown and bacterial decay. It must be done promptly after removal of tissue.
Which of the following is NOT an aim of fixation?
Water removal is the job of dehydration with alcohol. Fixation stops enzymatic and bacterial breakdown, preserves morphology and toughens tissue for later steps.
A resection left overnight in saline shows loss of nuclear detail and cell separation. Which step was delayed?
Without prompt fixation, tissue enzymes cause autolysis. The specimen should be placed in adequate fixative as soon as it is removed.
Routine formalin fixation of surgical specimens is normally carried out at:
Fixation is usually performed at ambient temperature. Heat speeds fixation but increases autolysis and shrinkage; some processors use mild warmth only.
Stock 37–40% formaldehyde solution is supplied with about 10% of an added alcohol that slows polymerization to paraformaldehyde. Which alcohol is it?
Methanol is added as a stabilizer to retard polymerization. Paraformaldehyde is the white polymer that forms when stabilization fails, not the stabilizer itself.
To prepare 10% formalin from 80 mL of concentrated (37–40%) formaldehyde, what volume of water is added?
10% formalin is a 1-in-10 dilution of the stock: 1 part stock plus 9 parts water, so 80 × 9 = 720 mL water (800 mL total).
How much water is added to 300 mL of 37–40% formaldehyde to prepare 10% formalin?
10% formalin is 1 part stock to 9 parts water, so 300 × 9 = 2700 mL water (3000 mL total).
Why is formalin stronger than 10% not used for routine fixation?
Stronger formalin overhardens tissue and causes more artifact, while 10% gives adequate fixation.