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AMCA MHTC practice questions

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Question 1Specimen Collection, Handling, and Fixation

Which needle gauge is the standard choice for routine adult venipuncture when a large-volume tube set is ordered?

  1. 18 gauge
  2. 16 gauge
  3. 25 gauge
  4. 21 gauge Correct answer.

Explanation

A 21-gauge needle is the routine adult venipuncture needle because it balances blood flow rate with vein trauma and hemolysis risk. Larger gauges are reserved for rapid large-volume collection or donation, and smaller gauges suit fragile or pediatric veins.

Question 2Specimen Collection, Handling, and Fixation

The recommended order of draw for a light blue sodium citrate tube relative to other tubes is which position?

  1. Last, after all serum tubes
  2. First, before any additive tube Correct answer.
  3. Second, after the serum tube
  4. Any position, since citrate tubes are unaffected by carryover

Explanation

Light blue citrate tubes are drawn first because additive carryover from other tubes can alter the citrate-to-blood ratio and invalidate coagulation results. When a dedicated coagulation tube is not available, a discard tube may precede it.

Question 3Specimen Collection, Handling, and Fixation

A phlebotomist must collect a specimen for blood alcohol testing. Which antiseptic is appropriate for skin preparation?

  1. Isopropyl alcohol
  2. Povidone-iodine Correct answer.
  3. Ethyl alcohol swab
  4. Chlorhexidine gluconate

Explanation

Alcohol-based antiseptics can contaminate the specimen and falsely elevate measured alcohol levels, so a non-alcoholic antiseptic such as povidone-iodine is used. Chlorhexidine is also non-alcoholic but is not the standard blood culture or alcohol-testing skin prep in most protocols.

Question 4Specimen Collection, Handling, and Fixation

Which condition most commonly causes a specimen to be rejected because of hemolysis?

  1. Filling the tube to the indicated volume line
  2. Mixing the tube by gentle inversion immediately after collection
  3. Drawing through a small-gauge needle with vigorous syringe aspiration Correct answer.
  4. Allowing the tube to clot upright at room temperature

Explanation

Mechanical trauma such as small-bore needles, excessive suction, or forceful ejection of blood through the needle shears red cells and releases hemoglobin. Proper fill volume, gentle inversion, and upright clotting do not cause hemolysis.

Question 5Specimen Collection, Handling, and Fixation

Blood cultures are ordered on an adult patient. What is the recommended collection approach?

  1. Collect all bottles from a single venipuncture after antibiotics are started
  2. Collect two sets, each from a separate site, before antimicrobial therapy Correct answer.
  3. Collect one anaerobic bottle only from the line port
  4. Collect a single aerobic bottle from one site

Explanation

Two sets from separate sites increase sensitivity and help distinguish contamination from true bacteremia, and collection should precede antimicrobial dosing when possible. Single bottles or post-therapy draws reduce recovery of organisms.

Question 6Specimen Collection, Handling, and Fixation

A capillary puncture is planned for an infant's heel. Which site is acceptable?

  1. The posterior curvature of the heel
  2. The tip of any toe
  3. The central area of the heel pad
  4. The medial or lateral plantar surface of the heel Correct answer.

Explanation

Heel punctures are performed on the medial or lateral plantar surface where tissue depth avoids the calcaneus and nerves. The central heel pad risks bone injury, and toes are not used in infants.

Question 7Specimen Collection, Handling, and Fixation

Which additive tube is used to collect a specimen for a complete blood count with differential?

  1. EDTA (lavender) Correct answer.
  2. Serum separator (gold)
  3. Sodium citrate
  4. Sodium heparin (green)

Explanation

EDTA chelates calcium and preserves cellular morphology, making it the anticoagulant of choice for hematology counts and differentials. Citrate dilutes the sample, heparin can alter staining, and serum separator tubes yield clotted serum without cells.

Question 8Specimen Collection, Handling, and Fixation

A specimen for plasma ammonia is collected. Which handling step is most critical to result validity?

  1. Store the whole blood at 37 degrees Celsius
  2. Transport on ice and separate plasma promptly Correct answer.
  3. Expose the tube to light during transport
  4. Allow the tube to clot at room temperature for one hour

Explanation

Ammonia rises in whole blood as red cells and proteins break down, so the specimen must be chilled and the plasma separated promptly. Warmth and delayed separation falsely elevate results.

Question 9Specimen Collection, Handling, and Fixation

Which tube fill condition is required for a sodium citrate coagulation tube to yield a valid result?

  1. Underfilled to allow mixing space
  2. Filled to half the marked volume
  3. Filled to the marked volume line Correct answer.
  4. Overfilled beyond the line

Explanation

Citrate tubes must be filled to the indicated volume to maintain the correct blood-to-anticoagulant ratio. Underfilling or overfilling changes that ratio and produces inaccurate clotting times.

Question 10Specimen Collection, Handling, and Fixation

A blood film for malaria parasites is being prepared. Which timing rule applies to the collection?

  1. Collect immediately after antimalarial therapy
  2. Collect only from capillary blood
  3. Collect at any time, since parasites are continuously present in blood Correct answer.
  4. Collect only during a documented fever spike

Explanation

Parasite levels fluctuate and may not coincide with fever, so smears are collected without regard to fever timing, often with repeat collections. Waiting for a fever spike can miss the diagnosis.

Question 11Specimen Collection, Handling, and Fixation

Which practice best prevents backflow of tube additives into the patient's vein during venipuncture?

  1. Removing the tube while the tourniquet is applied
  2. Keeping the patient's arm bent after the draw
  3. Shaking the tube vigorously during collection
  4. Holding the tube below the puncture site and releasing the tourniquet before removing the last tube Correct answer.

Explanation

Backflow is prevented by keeping the collection device below the venipuncture site and releasing the tourniquet before the final tube is removed. Arm bending and vigorous shaking do not prevent reflux and can cause other problems.

Question 12Specimen Collection, Handling, and Fixation

A 24-hour urine collection is being evaluated. Which finding indicates the collection should be rejected?

  1. The container was kept refrigerated during collection
  2. The container was labeled with start and end times
  3. The total volume was recorded on the container
  4. A portion of the first morning void was included in the collection Correct answer.

Explanation

A 24-hour collection begins after discarding the first morning void and includes all subsequent urine through the final void at the end time. Including the initial void adds urine from before the collection period and invalidates the result.

Question 13Specimen Collection, Handling, and Fixation

Which transport condition is correct for a routine serum chemistry specimen collected in a gel separator tube?

  1. Transport at room temperature and centrifuge within the recommended time Correct answer.
  2. Freeze whole blood immediately
  3. Expose to direct sunlight to prevent clotting
  4. Store at 37 degrees Celsius until analysis

Explanation

Serum separator specimens are typically transported at room temperature and centrifuged within the manufacturer's stated window so serum contacts cells for a limited time. Freezing whole blood, warming, or sunlight exposure degrades analytes.

Question 14Specimen Collection, Handling, and Fixation

A tissue specimen for routine histopathology is placed in 10% neutral buffered formalin. What is the primary purpose of this fixative?

  1. To lyse red blood cells
  2. To preserve cellular morphology by cross-linking proteins Correct answer.
  3. To provide a carbon source for organisms
  4. To decalcify bone

Explanation

Formalin cross-links proteins and preserves tissue architecture for microscopic examination. It is not a decalcifying agent and does not serve nutritional or hemolytic purposes.

Question 15Specimen Collection, Handling, and Fixation

The recommended ratio of 10% neutral buffered formalin to tissue volume for routine fixation is approximately which?

  1. 50:1
  2. 2:1
  3. 10:1 Correct answer.
  4. 1:1

Explanation

A fixative-to-tissue volume ratio of about 10:1 ensures adequate penetration and prevents fixative exhaustion by tissue fluids. Lower ratios can leave central tissue underfixed.

Question 16Specimen Collection, Handling, and Fixation

A specimen for electron microscopy requires which type of fixative?

  1. Glutaraldehyde-based fixative Correct answer.
  2. Zenker solution
  3. 10% neutral buffered formalin
  4. Bouin solution

Explanation

Electron microscopy requires glutaraldehyde, often combined with osmium tetroxide post-fixation, to preserve fine ultrastructure. Formalin and other light-microscopy fixatives do not preserve membranes and organelles adequately.

Question 17Specimen Collection, Handling, and Fixation

A tissue specimen must be transported fresh for frozen section. Which handling is appropriate?

  1. Fix in alcohol for one hour before transport
  2. Store at room temperature overnight
  3. Immerse in formalin
  4. Place on saline-moistened gauze on ice and transport immediately Correct answer.

Explanation

Frozen section specimens must remain unfixed, so they are kept moist with saline and chilled to limit degradation during rapid transport. Formalin or alcohol fixation prevents frozen section evaluation.

Question 18Specimen Collection, Handling, and Fixation

Which specimen type is preferred for detecting a suspected bacterial infection of a skin ulcer?

  1. A swab of surrounding intact skin
  2. A superficial swab of the exudate
  3. Tissue obtained from the ulcer base after cleaning Correct answer.
  4. A blood culture alone

Explanation

Tissue from the cleaned ulcer base yields organisms from the actual infected tissue, whereas superficial swabs often recover colonizing flora. Swabbing intact skin or relying on blood culture alone is less sensitive for a localized ulcer.

Question 19Specimen Collection, Handling, and Fixation

A sputum specimen is ordered for bacterial culture. Which specimen is acceptable for processing?

  1. Saliva collected after rinsing the mouth
  2. A 24-hour pooled collection
  3. An early-morning deep-cough specimen Correct answer.
  4. A nasal swab

Explanation

An early-morning deep-cough specimen provides lower respiratory secretions suitable for culture. Saliva is contaminated with oral flora, nasal swabs sample a different site, and pooled collections are not standard for bacterial sputum culture.

Question 20Specimen Collection, Handling, and Fixation

Which factor most strongly affects the concentration of a substance in a random urine specimen?

  1. The time of day the label was printed
  2. The patient's hydration status Correct answer.
  3. The size of the collection cup
  4. The color of the collection container

Explanation

Random urine concentration varies with fluid intake and hydration, which is why results are often normalized to creatinine. Container color, label timing, and cup size do not affect analyte concentration.

Question 21Specimen Collection, Handling, and Fixation

A blood specimen for glucose is collected in a tube containing sodium fluoride. What is the purpose of this additive?

  1. To preserve cell morphology
  2. To activate clotting
  3. To anticoagulate by chelating calcium
  4. To inhibit glycolysis and preserve glucose Correct answer.

Explanation

Fluoride inhibits the glycolytic enzyme enolase, slowing glucose consumption by cells. It is not an anticoagulant, morphology preservative, or clotting activator; oxalate is often paired with it for anticoagulation.

Question 22Specimen Collection, Handling, and Fixation

A newborn screening specimen is being collected on filter paper. Which practice is required?

  1. Allow blood to saturate the circle completely from one side Correct answer.
  2. Apply blood with a capillary tube touching the paper edge
  3. Layer multiple drops to fill the circle
  4. Apply blood to both sides of the paper

Explanation

Newborn screening cards require a single application of blood that saturates the circle from one side to ensure uniform absorption. Applying to both sides, layering drops, or touching the paper with a tube produces uneven or invalid spots.

Question 23Specimen Collection, Handling, and Fixation

Which action is correct when a patient reports fainting during a venipuncture?

  1. Have the patient stand and walk to another room
  2. Stop the draw, lower the patient's head, and apply a cold compress Correct answer.
  3. Continue the draw and reassure the patient
  4. Raise the patient's head above the heart

Explanation

For syncope, the draw should be stopped and the patient positioned to promote cerebral perfusion, such as lowering the head, with a cool compress and monitoring. Standing or raising the head worsens cerebral hypoperfusion.

Question 24Specimen Collection, Handling, and Fixation

Which specimen container is appropriate for a 24-hour urine collection for catecholamines?

  1. An amber container with acid preservative Correct answer.
  2. A sterile culture cup
  3. A clear container without preservative
  4. A serum separator tube

Explanation

Catecholamines are light-sensitive and require an amber container, often with acid preservative to stabilize the analytes. Clear containers, culture cups, and serum tubes do not meet these requirements.

Question 25Specimen Collection, Handling, and Fixation

A specimen for coagulation testing arrives in a citrate tube that was drawn from a heparin line without flushing. What is the most likely consequence?

  1. No effect, since citrate neutralizes heparin
  2. Falsely shortened clotting times from heparin contamination
  3. Falsely elevated platelet count
  4. Falsely prolonged clotting times from heparin contamination Correct answer.

Explanation

Residual heparin from an unflushed line prolongs clotting times and can mimic a coagulopathy. Citrate does not neutralize heparin, and platelet counts are not the primary concern in this scenario.

Question 26Specimen Collection, Handling, and Fixation

A blood specimen is collected in a tube containing EDTA. Which mechanism explains why this specimen remains suitable for hematology testing?

  1. EDTA lyses red cells to release intracellular contents
  2. EDTA binds calcium, blocking the coagulation cascade Correct answer.
  3. EDTA neutralizes heparin already present in the sample
  4. EDTA activates thrombin to speed clot formation

Explanation

EDTA is a chelating anticoagulant that sequesters calcium ions, which are required at multiple steps of the coagulation cascade; without ionized calcium the blood does not clot. Because it preserves cell morphology, EDTA whole blood is suited to hematology testing.

Question 27Specimen Collection, Handling, and Fixation

An order requires a serum specimen for a chemistry panel. Which collection and processing sequence yields serum?

  1. Collect in a tube with clot activator and separate the liquid after clotting Correct answer.
  2. Collect in a tube with heparin and centrifuge without waiting
  3. Collect in a tube with EDTA and centrifuge immediately
  4. Collect in a tube with sodium citrate and chill before centrifuging

Explanation

Serum is the liquid remaining after whole blood has clotted, so a serum tube contains clot activator (and often a gel separator) and the specimen must be allowed to clot before centrifugation. Anticoagulant tubes yield plasma, not serum.

Question 28Specimen Collection, Handling, and Fixation

A blood specimen is drawn into a tube containing sodium citrate for a routine coagulation test. Which condition is required for the result to be valid?

  1. The tube is filled to the marked volume so the blood-to-additive ratio is correct Correct answer.
  2. The tube is centrifuged before the draw is completed to save time
  3. The tube is filled only halfway to concentrate the anticoagulant
  4. The tube is shaken vigorously to mix the additive thoroughly

Explanation

Citrate tubes are calibrated so that a specific blood-to-anticoagulant ratio is achieved when filled to the marked volume. Underfilling leaves excess citrate relative to calcium, which can prolong clotting times and invalidate results.

Question 29Specimen Collection, Handling, and Fixation

A urine specimen for routine culture is collected from a patient using a clean-catch midstream technique. Which handling step best preserves the validity of the culture result?

  1. Collect the specimen into a container that previously held stool
  2. Add a preservative to the container to sterilize any bacteria present
  3. Store the specimen at room temperature for several hours before transport
  4. Refrigerate the specimen promptly and deliver it within the acceptable time window Correct answer.

Explanation

Urine supports bacterial growth at room temperature, so prompt refrigeration slows multiplication of contaminants and better reflects the in vivo population. Specimens should reach the laboratory within the established time limit to avoid overgrowth and false results.

Question 30Specimen Collection, Handling, and Fixation

A laboratory receives a blood specimen for potassium measurement that was collected in a tube with a gel separator but was transported without centrifugation for several hours. Which preanalytical effect is most likely to falsely elevate the result?

  1. Potassium binding to the gel separator and being released at analysis
  2. Potassium being consumed by clotting factors in the tube
  3. Potassium evaporating from the tube during transport
  4. Potassium leaking from cells into the serum or plasma during prolonged contact Correct answer.

Explanation

Potassium is present at much higher concentration inside cells than in plasma, so prolonged contact of cells with the liquid portion allows leakage that falsely raises the measured value. Prompt separation of serum or plasma from cells minimizes this preanalytical error.

Question 31Tissue Processing and Embedding

Which reagent is most commonly used as the clearing agent in routine paraffin processing of tissue?

  1. Ethanol
  2. Paraffin wax
  3. Xylene Correct answer.
  4. Formalin

Explanation

Clearing agents must be miscible with both the dehydrating agent and the embedding medium. Xylene is the standard clearing agent because it is miscible with alcohol and with molten paraffin. Formalin is a fixative, ethanol is a dehydrant, and paraffin is the embedding medium.

Question 32Tissue Processing and Embedding

What is the primary purpose of dehydration in tissue processing?

  1. To dissolve lipids and remove fat
  2. To support the tissue for microtomy
  3. To remove water so the tissue becomes miscible with clearing agents Correct answer.
  4. To cross-link proteins and harden the tissue

Explanation

Dehydration removes water from the tissue using graded alcohols so that the tissue becomes miscible with the clearing agent, which is not water-miscible. Cross-linking is fixation, lipid removal is clearing, and support is provided by embedding.

Question 33Tissue Processing and Embedding

Which embedding medium is typically used when sectioning requires a hard block and thin sections for routine H&E staining?

  1. Gelatin
  2. Paraffin wax Correct answer.
  3. OCT compound
  4. Agar

Explanation

Paraffin wax is the standard embedding medium for routine histology because it provides firm support for thin sectioning and is compatible with routine staining. Agar, OCT, and gelatin are used for other purposes such as frozen sections or fragile tissue support.

Question 34Tissue Processing and Embedding

During processing, which step immediately follows clearing in the standard sequence?

  1. Staining with hematoxylin
  2. Infiltration with molten paraffin Correct answer.
  3. Dehydration in graded alcohols
  4. Fixation in formalin

Explanation

The standard processing sequence is fixation, dehydration, clearing, and infiltration with the embedding medium. Infiltration with molten paraffin follows clearing because the cleared tissue is miscible with paraffin.

Question 35Tissue Processing and Embedding

What is the main consequence of incomplete dehydration before clearing?

  1. Loss of nuclear staining
  2. Increased tissue pigmentation
  3. A cloudy clearing agent and poor paraffin infiltration Correct answer.
  4. Excessive hardening of the tissue

Explanation

Residual water in the tissue reacts with the clearing agent to produce cloudiness and prevents proper paraffin infiltration. Incomplete dehydration does not cause nuclear staining loss or pigmentation.

Question 36Tissue Processing and Embedding

A tissue processor uses a vacuum during paraffin infiltration. What is the main benefit of this?

  1. It accelerates fixation of the tissue
  2. It removes air and improves penetration of paraffin into the tissue Correct answer.
  3. It prevents tissue from shrinking
  4. It sterilizes the tissue

Explanation

Vacuum-assisted infiltration removes trapped air and encourages molten paraffin to penetrate dense tissue more completely. It does not perform fixation, prevent shrinkage, or sterilize tissue.

Question 37Tissue Processing and Embedding

Which factor most increases the risk of tissue becoming brittle and difficult to section?

  1. Use of a vacuum during infiltration
  2. A slightly lower paraffin temperature
  3. Prolonged exposure to high-concentration alcohol Correct answer.
  4. Insufficient clearing time

Explanation

Overexposure to high-concentration alcohols over-dehydrates and hardens tissue, making it brittle. Insufficient clearing and lower paraffin temperature tend to cause poor infiltration rather than brittleness, and vacuum generally improves infiltration.

Question 38Tissue Processing and Embedding

What is the purpose of an orientation step when embedding a tissue specimen?

  1. To stain the tissue before sectioning
  2. To cool the block rapidly
  3. To remove excess paraffin from the block
  4. To position the tissue so the desired plane of section is presented to the blade Correct answer.

Explanation

Orientation ensures the diagnostic surface of the tissue is aligned so that the microtome blade cuts the intended plane. It is not about removing wax, cooling, or staining.

Question 39Tissue Processing and Embedding

Which embedding technique is most appropriate for a small, fragile biopsy that could be lost during processing?

  1. Processing without any support
  2. Freezing without any medium
  3. Embedding directly in paraffin without support
  4. Embedding within agar or a similar support medium before processing Correct answer.

Explanation

Small or fragile specimens are often supported in agar or a similar medium to prevent loss and distortion during processing. Processing without support risks losing the specimen, and freezing without medium does not provide support.

Question 40Tissue Processing and Embedding

What is the typical temperature range for molten paraffin used in embedding?

  1. 90 to 100 degrees Celsius
  2. 56 to 60 degrees Celsius Correct answer.
  3. 37 to 40 degrees Celsius
  4. 20 to 25 degrees Celsius

Explanation

Paraffin with a melting point around 56 to 60 degrees Celsius is standard, and it is kept molten slightly above its melting point during embedding. Room temperature would solidify it, and 90 to 100 degrees Celsius would damage tissue.

Question 41Tissue Processing and Embedding

A block shows a clear halo of paraffin around the tissue with poor infiltration at the edges. Which processing problem is most likely?

  1. Insufficient infiltration time or inadequate clearing Correct answer.
  2. Excessive trimming of the block
  3. Excessive fixation time
  4. Over-staining with hematoxylin

Explanation

Poor infiltration at the tissue edges suggests the paraffin did not fully penetrate, often due to inadequate clearing or insufficient infiltration time. Over-fixation, staining, and trimming do not produce this appearance.

Question 42Tissue Processing and Embedding

Which of the following best describes the role of a clearing agent in tissue processing?

  1. It supports the tissue during sectioning
  2. It replaces the dehydrant with a fluid miscible with the embedding medium Correct answer.
  3. It removes water from the tissue
  4. It fixes proteins in the tissue

Explanation

Clearing agents are miscible with both the dehydrating agent and the embedding medium, acting as a bridge between them. Removing water is dehydration, fixing proteins is fixation, and supporting tissue is embedding.

Question 43Tissue Processing and Embedding

Why is graded alcohol used rather than absolute alcohol alone for dehydration?

  1. To minimize tissue shrinkage and distortion Correct answer.
  2. To increase paraffin melting point
  3. To dissolve lipids
  4. To speed up fixation

Explanation

Gradual dehydration through increasing alcohol concentrations reduces osmotic shock and minimizes shrinkage and distortion. Absolute alcohol alone can cause abrupt shrinkage. It does not affect fixation, lipid dissolution, or paraffin melting point.

Question 44Tissue Processing and Embedding

What is the main reason for cooling an embedded block before trimming?

  1. To remove residual clearing agent
  2. To solidify the paraffin so the block holds its shape Correct answer.
  3. To sterilize the block
  4. To improve staining quality

Explanation

Cooling solidifies the paraffin so the block becomes firm and holds its shape for trimming and sectioning. Cooling does not affect staining, remove clearing agent, or sterilize.

Question 45Tissue Processing and Embedding

A tissue processor's paraffin bath is set well above the recommended temperature. What is the most likely effect on the tissue?

  1. Improved antigen preservation
  2. Reduced tissue shrinkage
  3. Increased water content
  4. Hardening and shrinkage of the tissue Correct answer.

Explanation

Excessive heat during infiltration can harden and shrink tissue and may damage it. It does not improve antigen preservation or reduce shrinkage, and it does not increase water content.

Question 46Tissue Processing and Embedding

Which statement about the use of xylene substitutes is correct?

  1. They are generally less toxic but may require longer clearing times Correct answer.
  2. They act as dehydrating agents
  3. They are always more toxic than xylene
  4. They cannot be used with paraffin

Explanation

Xylene substitutes are often chosen for lower toxicity but may need longer clearing times to achieve comparable results. They are not more toxic, they are compatible with paraffin, and they are clearing agents rather than dehydrants.

Question 47Tissue Processing and Embedding

What is the purpose of trimming a paraffin block before sectioning?

  1. To melt the paraffin
  2. To expose the full face of the tissue at the desired level Correct answer.
  3. To stain the tissue
  4. To remove the tissue from the block

Explanation

Trimming removes excess paraffin until the full tissue face is exposed at the desired level for sectioning. It does not remove the tissue, stain it, or melt the paraffin.

Question 48Tissue Processing and Embedding

Which of the following is a consequence of inadequate fixation before processing?

  1. Poor morphology and uneven staining Correct answer.
  2. Improved tissue sectioning
  3. Better antigen preservation
  4. Reduced tissue shrinkage

Explanation

Inadequate fixation leaves tissue soft and poorly preserved, leading to poor morphology and uneven staining. It does not improve sectioning, reduce shrinkage, or improve antigen preservation.

Question 49Tissue Processing and Embedding

Which embedding medium is most suitable for frozen sections that will be cut immediately?

  1. Agar
  2. Celloidin
  3. Paraffin wax
  4. OCT compound Correct answer.

Explanation

OCT compound is a cryo-embedding medium that supports tissue for frozen sectioning. Paraffin and celloidin require processing steps and are not used for immediate frozen sections, and agar is a support medium not designed for cryotomy.

Question 50Tissue Processing and Embedding

What is the most likely cause of a paraffin block that crumbles during sectioning?

  1. Excessive trimming
  2. Incomplete infiltration or paraffin that is too cold Correct answer.
  3. Excessive clearing time
  4. Over-staining of the tissue

Explanation

Crumbling blocks often result from incomplete paraffin infiltration or from cutting when the block is too cold. Excessive clearing, staining, and trimming do not cause crumbling.