A 45-year-old man is brought to the hospital by his daughter with complaints of wide-based gait. His daughter reveals that he was sitting silently in the examination chair with a blank face. In addition, he was frequently talking to the empty chairs and told that his friends are sitting there. He has been forgetting many small things recently. On physical examination, fine movements are seen at resting condition that disappears when he is asked to drink water. A stepwise slowness in movement is also seen in his upper limb. Which of the following is most likely to be observed in the histological specimen of this patient?

Lewy bodies in affected neurons

Tau protein aggregates in cortex

Neurofibrillary tangles in hippocampus

A 36-year-old woman is brought to the emergency room by her husband for “weird behavior" for the past several weeks. He reports that her right arm has been moving uncontrollably in a writhing movement and that she has been especially irritable. She has a history of depression, which was diagnosed 4 years ago and is currently being treated with sertraline. She denies any recent fever, trauma, infections, travel, weakness, or sensory changes. She was adopted so is unsure of her family history. Which of the following is the most likely explanation for her symptoms?

CAG triplet expansion on chromosome 4

Frontotemporal lobe degeneration

Presence of misfolded proteins in the brain

Development of intracellular eosinophilic inclusions

GAA triplet expansion on chromosome 9

A 1-week-old baby is brought to the pediatrician’s office for a routine checkup. On examination, she is observed to have microcephaly with a prominent occiput. She also has clenched fists and rocker-bottom feet with prominent calcanei. A cardiac murmur is evident on auscultation. Based on the clinical findings, a diagnosis of nondisjunction of chromosome 18 is suspected. The pediatrician orders a karyotype for confirmation. He goes on to explain to the mother that her child will face severe growth difficulties. Even if her daughter progresses beyond a few months, she will not be able to reach developmental milestones at the appropriate age. In addition to the above, which of the following is most likely a consequence of this genetic disturbance?

Death within the first year of life

A 68-year-old man is brought to the physician by his wife because she is concerned about his speech being irregular. Specifically, she says that over the last 8 months, her husband has been saying increasingly nonsensical statements at home. In addition, he is no longer able to perform basic verbal tasks such as ordering from a menu or giving directions even though he was an English teacher prior to retirement. She also reports that he has recently started attempting to kiss strangers and urinate in public. Finally, she has also noticed that he has been frequently binge eating sweets even though he was previously very conscientious about his health. When asked about these activities, the patient does not have insight into his symptoms. Which of the following would most likely be seen in this patient?

Hyperphosphorylated tau inclusion bodies

Intracellular hyperphosphorylated tau proteins

A 69-year-old man is brought to clinic by his daughter for poor memory. She states that over the past two years his memory has been slowly declining though he has been able to take care of himself, pay his own rent, and manage his finances. However, two months ago she noticed a sharp decline in his cognitive functioning as well as his gait. Then one month ago, she noticed a similar decline in his functioning again that came on suddenly. The patient has a past medical history of diabetes mellitus type II, hypertension, obesity, and dyslipidemia. Current medications include hydrochlorothiazide, lisinopril, metformin, and glipizide. His blood pressure is 165/95 mmHg, pulse is 82/minute, he is afebrile, and oxygen saturation is 98% on room air. Cardiac exam reveals a crescendo-decrescendo murmur heard in the left upper sternal border that radiates to the carotids. Abdominal exam is benign, and neurologic exam reveals an unsteady gait. Which of the following findings is associated with the most likely diagnosis?

Neurofibrillary tangles and hyperphosphorylated tau

Diffuse, subtle atrophy of the brain, subtle ventricular enlargement

Alzheimer's disease pathology for USMLE Step 3: High-Yield Pathology Lessons

Gross Pathology - Shrinking Brain Blues

Normal vs. Alzheimer's Brain: Gross and Sectional Views

Generalized cortical atrophy: The brain shrinks, leading to:
- Widened sulci: The grooves on the brain's surface become broader.
- Narrowed gyri: The folds of the brain become thinner.
Hydrocephalus ex vacuo: Compensatory enlargement of ventricles as brain parenchyma is lost. CSF pressure remains normal.
Hippocampal atrophy: Severe shrinkage of the hippocampus is a key feature.

⭐ Hippocampal atrophy is a key early finding on MRI, correlating with the severity of memory loss.

Extracellular Pathology - Plaque Attack

Senile (neuritic) plaques are extracellular aggregates of Amyloid-β (Aβ) peptides, primarily the toxic Aβ42 fragment, formed by enzymatic cleavage of Amyloid Precursor Protein (APP).

Staining: Visualized with Congo Red (apple-green birefringence) and Thioflavin-S.
Cerebral Amyloid Angiopathy (CAA): Aβ deposition in vessel walls, increasing hemorrhage risk.

⭐ Aβ42's hydrophobic nature makes it highly prone to aggregation, initiating plaque formation.

Intracellular Pathology - Tangled Tau

Normal Function: Tau protein is an intracellular microtubule-associated protein (MAP) that provides structural stability to neuronal axons, acting like railway ties for axonal transport.
Pathology: In Alzheimer's disease, Tau becomes pathologically hyperphosphorylated.
- This causes Tau to detach from microtubules, leading to their destabilization and disintegration.
- The abnormal Tau protein misfolds and aggregates into insoluble Paired Helical Filaments (PHFs).
Result - NFTs: These PHFs accumulate within the neuron to form Neurofibrillary Tangles (NFTs).
- Appearance: NFTs have a classic intracytoplasmic, "flame-shaped" appearance in pyramidal neurons.
- Staining: Visualized with silver stains (Bielschowsky, Gallyas) and immunohistochemistry for Tau.

Alzheimer's pathology: plaques, tangles, and protein stains

⭐ High-Yield: Tangle density and distribution (measured by Braak staging) correlate much more strongly with the severity of cognitive decline in Alzheimer's disease than amyloid plaque load.

Genetic Factors - The Gene Scene

Onset Type	Key Genes & Loci	Notes
Late-Onset (>95%)	ApoE4 allele on Chr 19	↑ Risk for sporadic AD
(Sporadic)	ApoE2 allele on Chr 19	↓ Protective
Early-Onset (<5%)	APP on Chr 21	Amyloid Precursor Protein
(Familial)	PSEN1 on Chr 14	Presenilin 1 (most common)
	PSEN2 on Chr 1	Presenilin 2

⭐ The ApoE4 allele is the strongest genetic risk factor for late-onset AD.

High‑Yield Points - ⚡ Biggest Takeaways

Core pathology involves extracellular Aβ-amyloid plaques (derived from APP) and intracellular neurofibrillary tangles of hyperphosphorylated tau.

Gross anatomy shows diffuse cortical atrophy, most prominent in the hippocampus and temporoparietal lobes.

Cerebral amyloid angiopathy (CAA) is a frequent co-pathology, elevating the risk of lobar hemorrhage.

ApoE4 allele is the most significant genetic risk factor for late-onset AD.

Early-onset familial AD is linked to mutations in APP, PSEN1, and PSEN2.

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