A 78-year-old man dies suddenly from complications of acute kidney failure. An autopsy is performed and microscopic evaluation of the kidneys shows pale, swollen cells in the proximal convoluted tubules. Microscopic evaluation of the liver shows similar findings. Which of the following is the most likely underlying mechanism of these findings?

Impaired Na+/K+-ATPase pump activity

Cytoplasmic triglyceride accumulation

A 74-year-old man presents with complaints of sudden severe crushing retrosternal pain. The pain radiated to his left arm shortly after it began, and he was subsequently rushed to the emergency department for evaluation. His troponins and creatine kinase-MB (CK-MB) were elevated. Unfortunately, the patient died within the next 2 hours and an autopsy was performed immediately. The gross examination of the heart will show?

White, patchy, non-contractile scar

Red granulation tissue surrounding the infarction

A 72-year-old female is brought to the emergency department by ambulance because she was unable to walk. She says that she cut her leg while falling about a week ago. Since then, the wound has started draining fluid and become progressively more painful. She is found to have necrotizing fasciitis and is taken emergently to the operating room. Histological examination of cells along the fascial planes reveal cells undergoing necrosis. Which of the following represents the earliest sign that a cell has progressed to irreversible damage in this patient?

Condensation of DNA into a basophilic mass

Membrane blebbing from organelles

Chromatin dissolution and disappearance

Ribosomal detachment from the endoplasmic reticulum

A 78-year-old man with a history of myocardial infarction status post coronary artery bypass grafting and a 60-pack-year history of smoking is found deceased in his apartment after not returning calls to his family for the last 2 days. The man was last known to be alive 3 days ago, when his neighbor saw him getting his mail. The family requests an autopsy. On autopsy, the man is found to have a 100% blockage of his left anterior descending artery of his heart and likely passed from sudden cardiac death 2 days prior. Which of the following findings is expected to be found on histologic examination of his damaged myocardium?

Uniform binding of acidophilic dyes

Cellular debris and lymphocytes

Cellular debris and macrophages

A 31-year-old woman scrapes her finger on an exposed nail and sustains a minor laceration. Five minutes later, her finger is red, swollen, and painful. She has no past medical history and does not take any medications. She drinks socially with her friends and does not smoke. The inflammatory cell type most likely to be prominent in this patient's finger has which of the following characteristics?

Dramatically expanded endoplasmic reticulum

Large cell with amoeboid movement

Dark histamine containing granules

A 45-year-old man comes to his primary care provider for a routine visit. The patient mentions that while he was cooking 5 days ago, he accidentally cut himself with a meat cleaver and lost the skin at the tip of his finger. After applying pressure and ice, the bleeding stopped and he did not seek treatment. The patient is otherwise healthy and does not take any daily medications. The patient’s temperature is 98.2°F (36.8°C), blood pressure is 114/72 mmHg, pulse is 60/min, and respirations are 12/min. On exam, the patient demonstrates a 0.5 x 0.3 cm wound on the tip of his left third finger. No bone is involved, and the wound is red, soft, and painless. There are no signs of infection. Which of the following can be expected on histopathological examination of the wounded area?

Deposition of type III collagen

Epithelial cell migration from the wound borders

Neutrophil migration into the wound

A 10-year-old boy presents to your office with puffy eyes. The patient's mother states that his eyes seem abnormally puffy and thinks he may have an eye infection. Additionally, he had a sore throat a week ago which resolved with over the counter medications. The mother also thought that his urine was darker than usual and is concerned that blood may be present. His temperature is 99.5°F (37.5°C), blood pressure is 107/62 mmHg, pulse is 100/min, respirations are 17/min, and oxygen saturation is 98% on room air. Physical exam is notable for bilateral periorbital edema. Cranial nerves are grossly intact bilaterally. Which of the following is the most likely finding on renal biopsy for this patient?

Sub-epithelial electron dense deposits on electron microscopy

Linear Ig deposits along the basement membrane

Podocyte fusion on electron microscopy

Alternating thickening and thinning of basement membrane with splitting of the lamina densa

Histopathological changes in common diseases — USMLE Lesson

Cellular Adaptations & Injury - Cells Under Stress

Adaptation: Reversible changes in cell size, number, phenotype, or function in response to stress.
- Atrophy: ↓ cell size/number (e.g., disuse atrophy).
- Hypertrophy: ↑ cell size (e.g., cardiac hypertrophy).
- Hyperplasia: ↑ cell number (e.g., benign prostatic hyperplasia).
- Metaplasia: One adult cell type replaced by another (e.g., Barrett esophagus).
Cell Injury: Occurs when stress exceeds adaptive capacity.
- Reversible: Cellular swelling, membrane blebbing, ↓ ATP production.
- Irreversible: Severe membrane/mitochondrial damage, Ca²⁺ influx, lysosomal rupture → necrosis/apoptosis.

⭐ The hallmark of irreversible cell injury is massive calcium influx and the mitochondrial permeability transition, which prevents ATP synthesis and releases pro-apoptotic proteins.

Inflammation & Repair - The Body's Repair Crew

Cardinal Signs: Rubor (redness), Calor (heat), Tumor (swelling), Dolor (pain), Functio laesa (loss of function).
Acute: Rapid onset, short duration. Key cell: Neutrophil.
Chronic: Prolonged duration. Key cells: Monocytes/Macrophages, Lymphocytes.

Repair Outcomes:
- Regeneration: Replacement by native tissue; ideal outcome.
- Scarring (Fibrosis): Collagen deposition when severe damage precludes regeneration.

⭐ Granulation tissue is the hallmark of healing, featuring fibroblasts (depositing type III collagen) and new capillaries. This is later remodeled to a scar with stronger type I collagen.

Wound Healing: Granulation Tissue Formation and Remodeling

Neoplasia & Dysplasia - Good Cells Gone Bad

Dysplasia: Disordered, non-neoplastic growth; a prelude to cancer. Potentially reversible.
- Loss of cell uniformity (pleomorphism) & architectural orientation.
- Increased nuclear-to-cytoplasmic (N:C) ratio (normal 1:4 → 1:1).
- Nuclear hyperchromasia (darker staining) and clumped chromatin.
Neoplasia: Abnormal, autonomous new growth (tumor).
- Benign: Well-differentiated, localized, does not metastasize.
- Malignant: Invades, destroys, and metastasizes. Shows poor differentiation (anaplasia).

⭐ Anaplasia is the hallmark of malignancy. It signifies a complete lack of differentiation, with bizarre, pleomorphic cells and atypical, giant tumor cells.

Cervical Intraepithelial Neoplasia (CIN) Histology

Vascular & Organ Pathology - System-Specific Snapshots

Heart (Myocardial Infarction): Progression from coagulative necrosis and wavy fibers (4-24h) to neutrophil infiltration (1-3d), followed by macrophage-led debris removal and granulation tissue (3-14d).
Kidney (Diabetic Nephropathy): Kimmelstiel-Wilson lesions (nodular glomerulosclerosis), hyaline arteriolosclerosis (especially efferent arteriole), and thickened glomerular basement membranes.
Liver (Cirrhosis): Diffuse process with bridging fibrous septa, regenerative parenchymal nodules, and disruption of the entire liver architecture.
Lung (ARDS): Diffuse alveolar damage (DAD) is the hallmark, showing hyaline membrane formation lining alveolar ducts and spaces.

⭐ In ARDS, the characteristic hyaline membranes are composed of fibrin-rich edema fluid mixed with the cytoplasmic and lipid remnants of necrotic epithelial cells.

High‑Yield Points - ⚡ Biggest Takeaways

Myocardial infarction shows coagulative necrosis, then neutrophil infiltration, followed by macrophages.

Alzheimer's disease: Extracellular amyloid-β plaques and intracellular neurofibrillary tangles (hyperphosphorylated tau).

Parkinson's disease: Lewy bodies (eosinophilic inclusions) in the substantia nigra.

Cirrhosis: Regenerative hepatocyte nodules separated by bands of bridging fibrosis.

Barrett's esophagus: Intestinal metaplasia of esophageal mucosa with goblet cells.

Celiac disease: Villous atrophy, crypt hyperplasia, and increased intraepithelial lymphocytes.

Koilocytes are pathognomonic for HPV, showing perinuclear halos.

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