This article is the first of three posts that distil high yield ideas from the Pathology and Pharmacology chapters of First Aid for the USMLE Step 1 2026 (FA) and show how the two chapters reinforce each other at the exam level. It is meant as a roadmap and cross link, not a substitute for reading the full book chapters, which provide the complete tables, images, and details you’ll need for Step 1.

Introduction

When cells are stressed, they try to adapt. When the stress is too much, they get injured and die. Understanding that story at the cellular and tissue level makes a lot of pathology  easier to handle on Step 1.

When Cells Cope: Adaptation vs Early Injury

Cells have a small set of ways to “cope” with chronic stress before they give up.

  • Hypertrophy: increased cells size -  Example: Left ventricular thickening in long standing hypertension.
  • Hyperplasia: increased cell number - Example: glandular proliferation in the endometrium with excess estrogen.
  • Atrophy: loss of cell size and/or number - Example: in a limb immobilized in a cast
  • Metaplasia: reversible change in cell type - Example: replacement of squamous epithelium by intestinal type columnar epithelium in Barrett esophagus after chronic acid exposure (metaplasia → dysplasia → adenocarcinoma)

These changes are still controlled and potentially reversible. It matters because persistent stress can push metaplasia or pathologic hyperplasia into dysplasia, a reversible but precancerous pattern that can progress to carcinoma in situ and invasive cancer. In this blog, we’ll focus on patterns that keep showing up in vignettes and pharm tables.

In reversible injury, ATP falls, ion pumps fail, water enters the cell, causing swelling, membrane blebs, and clumped chromatin; after an MI, this necrotic myocardium then triggers the classic sequence of neutrophils, macrophages, granulation tissue, and scar you’ll recognize from the inflammation and healing sections of the pathology chapter (Pathology, p209). 

Once damage crosses a threshold, the injury becomes irreversible. Plasma membranes rupture, and in the nucleus, you see condensation (pyknosis), fragmentation (karyorrhexis), and dissolution (karyolysis). At this point, the cell is committed to death. Reperfusion after an MI can rescue some cells but also sets up the reperfusion injury you’ll see again in the free radical section below.

When Cells Die: Necrosis Patterns You Really Need

Necrosis is uncontrolled cell death with membrane rupture and inflammation – link each pattern to typical organs (coagulative in most infarcts, liquefactive in brain or abscesses).

Coagulative necrosis is the archetypal infarct pattern in solid organs (except the brain). Ischemia or infarction denatures structural proteins and enzymes, blocking proteolysis. Histologically, the overall cellular architecture is preserved (cell outlines remain) but nuclei disappear, and the cytoplasm becomes more eosinophilic. 

Liquefactive necrosis occurs when enzymatic digestion dominates, as in bacterial abscesses and CNS infarcts. Neutrophils empty lysosomal enzymes that digest tissue, leaving behind cellular debris and later cystic spaces filled with pus or fluid. In the brain, liquefactive necrosis from an infarct ultimately leaves cystic spaces and cavitation, rather than the firm scar typical of infarcts in other solid organs.

Caseous necrosis is most famous in tuberculosis and systemic fungal infections: amorphous granular debris with a cheesy (“caseous”) appearance within a granuloma; on exams, “caseating granuloma” should immediately point you to this pattern.

Also remember fat necrosis and fibrinoid necrosis as additional named patterns.

Free Radicals: A Unifying Mechanism for Toxic Injury

Free radicals are molecules with unpaired electrons, highly reactive with lipids, proteins, and DNA. They attack cell membranes through lipid peroxidation, alter proteins, and cause DNA strand breaks.

They arise in several clinically important ways:

  • Ionizing radiation (DNA breaks, radicals from water)
  • Phase I liver metabolism of drugs and chemicals
  • Redox reactions and nitric oxide during inflammation
  • Transition metals via Fenton reaction
  • Oxidative burst (neutrophils, macrophages)

The liver provides classic examples. Acetaminophen overdose leads to accumulation of a toxic metabolite. FA’s drug reaction table (Pharmacology, p248) lists acetaminophen under ‘focal to massive hepatic necrosis,’ that reflects its free radical mediated centrilobular necrosis, treated with N acetylcysteine to replenish glutathione. 

Carbon tetrachloride (CCl4) is converted by cytochrome P 450 to the CCl₃ radical, which initiates lipid peroxidation, resulting in fatty change and centrilobular necrosis. The free radical injury section in FA (Pathology, p206) pulls examples together (oxygen toxicity, reperfusion injury, hemochromatosis, Wilson disease). 

Cells counter free radicals using enzymatic systems, antioxidants, and metal binding proteins, defenses that FA revisits in the pharmacology discussions of drug metabolism, cytochrome P 450 interactions, and toxicity treatments (Pharmacology, p230). 

Pathologic Calcification: Dystrophic vs Metastatic

Dystrophic calcification is “abnormal tissue, normal labs”: calcium deposits in damaged or necrotic areas despite normal serum calcium and phosphate, typically in localized lesions such as calcific aortic stenosis, old TB granulomas, or atherosclerotic plaques, with psammoma bodies in certain tumors listed in the FA calcification table (Pathology, p207). On histology these deposits appear as deep basophilic (blue purple) granules.

Metastatic calcification is “normal tissue, abnormal labs”: calcium deposits in otherwise normal interstitial tissues of kidney, lung, or gastric mucosa because of hypercalcemia or hyperphosphatemia, classically from chronic kidney disease (CKD) with secondary hyperparathyroidism and high phosphate, or from primary hyperparathyroidism and related causes in the same table. Nephrocalcinosis in this setting can progress to nephrogenic diabetes insipidus and renal failure.

Clinically, a focal calcified aortic valve with normal calcium levels points to dystrophic calcification, whereas diffuse calcification in normal organs in a patient with CKD and elevated phosphate reflects metastatic calcification due to chronic hyperphosphatemia and secondary hyperparathyroidism. 

When you review vitamin D, calcium, and CKD therapies in pharmacology, you can relate their effects on calcium-phosphate balance back to this metastatic calcification framework.

Amyloidosis: Recognizing a Recurring Pattern

Amyloidosis is not a single disease but a pattern: extracellular deposition of misfolded proteins in an abnormal fibrillar, β pleated sheet configuration that disrupts organ function.

Systemic forms include:

  • AL (amyloid light chain): often in plasma cell dyscrasias like multiple myeloma.
  • AA (from serum amyloid A): seen in chronic inflammatory conditions.
  • Transthyretin (wild type form (in aging), or mutant (familial polyneuropathy and/or cardiomyopathy).
  • Dialysis related (from β2 microglobulin): end stage renal disease on long term dialysis.

FA’s amyloidosis table (Pathology, p208), also highlights localized deposits such as Aβ in Alzheimer disease and islet amyloid in type 2 diabetes. Amyloid shows up as amorphous pink material on routine hematoxylin and eosin, red orange on Congo red, and characteristic apple green birefringence under polarized light. 

Because AA amyloidosis arises from chronically elevated serum amyloid A in inflammatory diseases like rheumatoid arthritis and inflammatory bowel disease, any therapy that controls that inflammation indirectly reduces risk of AA deposition. The same logic applies behind disease-modifying antirheumatic and biologic anti-cytokine drugs in the Pharmacology chapter, which aim to dampen chronic inflammation at source.

Quick Check: Can You Tell the Story Back?

Try quizzing yourself with the following review questions below or in FA. If  you can see how each of these fits into the same timeline – adaptation, reversible injury, irreversible injury/necrosis, and sometimes abnormal deposits – and you can point to where the pharmacology chapter revisits the same mechanisms as drug toxicities and treatments, you’re turning two dense chapters into one coherent framework you can reuse throughout your Step 1 prep.

All these mechanisms lie at the start of the path to cancer, where chronic injury and inflammation promote mutations and dysplasia, and the next blog will follow that progression into tumor genes, grade and stage, and metastatic patterns.

Try answering these in your own words:

  1. A heart biopsy after an infarct shows preserved architecture but no nuclei. What type of necrosis is this, and why is the architecture maintained?
  2. A patient with long‑standing CKD and hyperphosphatemia has nephrocalcinosis in normal renal tissue. Is this dystrophic or metastatic calcification, and what lab pattern supports your choice?
  3. A child with an acetaminophen overdose develops centrilobular hepatic necrosis. How do free radicals and glutathione depletion fit into this picture, and which antidote from your pharmacology tables addresses it?
  4. A lung lesion in a patient with TB has a cheesy center surrounded by a granuloma. What kind of necrosis is that, and which other infections can produce it?

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