Pathology cheat sheet

Cause and mechanism

Aetiology asks what causes a disease. Pathogenesis describes the sequence through which it develops.

A cause is not the same thing as the tissue changes it produces.

Cell injury

Cells can adapt to stress. Injury may be reversible, or progress to irreversible damage and cell death.

Distinguish an adaptive response from evidence of tissue injury.

Inflammation

Inflammation is a tissue response to injury or infection involving immune cells, chemical signals and blood vessels.

Redness, warmth, swelling and pain are classical local features.

Neoplasia

A neoplasm is abnormal tissue growth. Benign and malignant describe different growth behaviour, rather than the size alone.

Invasion of surrounding tissue is an important malignant feature.

Adaptation versus injury

Hypertrophy increases cell size; hyperplasia increases cell number; atrophy reduces cell size or tissue mass. These are descriptive responses and are not interchangeable with cancer.

Muscle enlargement and an increased number of cells describe different mechanisms.

Cell-death patterns

Apoptosis is a regulated cell-death process, often involving cell shrinkage and clearance of fragments. Necrosis describes injury-associated cell death with loss of membrane integrity and local inflammation.

Morphological patterns help frame a mechanism; one microscopic feature rarely establishes the complete cause.

Acute and chronic inflammation

Acute inflammation often has a rapid vascular and cellular response. Chronic inflammation involves persistent injury, immune activity and repair; duration and cell populations must be interpreted in context.

Neutrophils are commonly prominent early in many acute inflammatory responses.

Repair and fibrosis

Tissue can recover through regeneration, scar formation or both. Outcome depends on surviving cells, the supporting tissue framework and whether the injurious process continues.

Collagen deposition restores structural continuity but may not restore the original tissue function.

Worked example: test accuracy

In a study of 100 people with a defined disease, a test detects 90. Sensitivity is 90 ÷ 100 = 90%. This calculation does not tell you the probability of disease after a positive test.

Sensitivity uses people with the condition as its denominator; predictive value uses people with a particular test result.

Common mistake: grade equals stage

Grade describes features such as differentiation or proliferative appearance. Stage describes disease extent using a disease-specific system. They answer different questions and cannot be substituted for each other.

A description of cellular appearance alone does not establish anatomical spread.

Check your understanding

Which denominator is used for sensitivity?

All people with the condition in the reference classification: true positives ÷ (true positives + false negatives).

Curriculum connections

Sources