Scientific Foundations

Application Questions

1. A 58-year-old woman has metastatic breast cancer. Her tumor cells express high levels of cyclin D. She starts palbociclib, a selective inhibitor of Cdk4/6, and her disease is stable for 14 months. Then new lesions appear. A biopsy of a new lesion shows the same cyclin D overexpression plus a new mutation in one gene. A loss-of-function mutation in which of the following genes would most likely explain why the drug stopped working?

   1. CDKN2A (encoding Ink4)
   2. MYC
   3. RB1 (encoding pRb)
   4. KRAS (encoding Ras)
   5. CCND1 (encoding cyclin D)

   Show Explanation

   Cyclin D–Cdk4 drives cells through Start by phosphorylating pRb. When pRb is phosphorylated, it releases E2F1–3 so they can activate cyclin E transcription. Palbociclib keeps pRb unphosphorylated, so pRb stays bound to E2F and the cell stops in G1. If pRb is lost, E2F is free no matter what Cdk4 is doing, so blocking Cdk4 does nothing.

   • A is wrong because losing Ink4 increases Cdk4 activity, and the drug still blocks Cdk4 directly.
   • B, D and E are oncogenes that act upstream of Cdk4, and gain of function in them is what usually drives the disease.
   • Losing function in an oncogene would not cause resistance.

2. A 64-year-old man has metastatic colorectal cancer that overexpresses EGFR, a receptor tyrosine kinase. His oncologist is thinking about cetuximab, an antibody that blocks EGF from binding EGFR. Molecular testing finds an activating mutation in codon 12 of KRAS, and the oncologist decides the drug will not help. What is the best explanation for this decision?

   1. The mutant Ras protein binds cetuximab and neutralizes it.
   2. The mutant Ras stays bound to GTP and keeps the MAP kinase pathway, Myc and cyclin D expression active without any signal from the receptor.
   3. The mutant Ras activates the TGF-β receptor, which blocks Smad signaling.
   4. The mutant Ras turns on p53, which makes the cells resistant to apoptosis.
   5. The mutant Ras stops EGFR from being made, so the drug has no target.

   Show Explanation

   Normally, when EGF binds its receptor, the receptor activates a GEF that loads Ras with GTP. Ras-GTP then turns on a MAP kinase cascade, which activates Myc, which increases transcription of cyclin D and pushes the cell into Start. An activating KRAS mutation locks Ras in the GTP-bound state, and that step sits downstream of the receptor. The cell keeps entering the cycle without any growth signal, which is a hallmark of cancer, so blocking the receptor has no effect.

   • D describes something real: oncogenic Ras turns on p14-ARF, which blocks Mdm2 and activates p53. But p53 activation slows the cell cycle and promotes apoptosis, so it does not cause resistance. This is the reason tumors usually also need a hit in a tumor suppressor (the two-hit model).
   • A, C and E do not match any mechanism in the pathway.

3. A research laboratory creates a mouse model with knockout of the gene encoding superoxide dismutase 1 (SOD1). Compared to wild-type mice, these knockout mice are expected to die early from ROS-mediated disease. Which of the following best describes the normal function of SOD1 and the consequence of its absence?

   1. SOD1 converts H₂O₂ to water; its absence allows H₂O₂ to accumulate and cause DNA strand breaks
   2. SOD1 converts superoxide anion (O₂^-) to H₂O₂; its absence causes superoxide accumulation and widespread oxidative damage
   3. SOD1 repairs 8-oxo-G DNA lesions; its absence leads to unrepaired mutations and early-onset cancer
   4. SOD1 activates catalase; its absence impairs the breakdown of hydroxyl radicals
   5. SOD1 refolds oxidized proteins; its absence leads to protein aggregation and neurodegeneration

   Show Explanation

   Explanation: SOD1 (and SOD2) catalyze the conversion of the superoxide anion (O₂^-) → hydrogen peroxide (H₂O₂), which is then further detoxified by catalase or glutathione peroxidase to water. Without SOD1, superoxide accumulates, causing widespread oxidative damage to lipids, proteins, and DNA. Notably, SOD1 mutations are also implicated in familial ALS, where toxic protein aggregates form in motor neurons — an important clinicopathologic correlation.

4. A 52-year-old man with a history of alcoholic cirrhosis is brought to the emergency department after ingesting a large quantity of acetaminophen in a suicide attempt. Lab work reveals markedly elevated AST and ALT. A liver biopsy is performed and shows hepatocyte swelling, membrane disruption, and leakage of intracellular contents into surrounding tissue. On further workup, it is found that toxic metabolites of acetaminophen depleted hepatocellular glutathione stores. Which of the following best explains the mechanism by which glutathione depletion leads to hepatocyte death?

   1. Decreased ATP production due to inhibition of the mitochondrial electron transport chain
   2. Direct inhibition of superoxide dismutase, causing superoxide anion accumulation
   3. Activation of HSP70 chaperones leading to ubiquitin-proteasome degradation of hepatocytes
   4. Loss of antioxidant defense allowing ROS accumulation, leading to lipid peroxidation and membrane disruption
   5. Impaired DNA repair via inhibition of XPA-mediated nucleotide excision repair

   Show Explanation

   Glutathione peroxidase uses glutathione (GSH) to neutralize H₂O₂ and lipid peroxides, converting them to water and non-toxic lipid alcohols. In acetaminophen toxicity, the toxic metabolite NAPQI rapidly depletes hepatocellular glutathione stores. Without adequate GSH, the antioxidant defense system is overwhelmed → ROS accumulate → lipid peroxidation of cell membranes → membrane disruption → leakage of intracellular contents → irreversible cell injury and necrosis. This follows the classic ROS oxidative damage cascade:

   $O_2\to O_2^{\bullet -}\to H_2{O}_2\to \bullet OH$

   This is why N-acetylcysteine (NAC) is the antidote for acetaminophen overdose — it replenishes glutathione stores, restoring antioxidant defenses before irreversible injury occurs.

5. A 58-year-old man with a 30-year history of smoking presents for a routine chest CT, which reveals thickened bronchial walls. Bronchoscopic biopsy shows replacement of the normal pseudostratified columnar epithelium with stratified squamous epithelium. There are no nuclear atypia or architectural disturbances. Which of the following best describes the underlying mechanism driving this change?

   1. Somatic mutations in tumor suppressor genes
   2. Activation of proto-oncogenes by carcinogens
   3. Epigenetic changes including DNA methylation and gene silencing
   4. Mitotic spindle dysfunction leading to aneuploidy
   5. Increased ROS production causing direct nuclear damage

   Show Explanation

   This describes squamous metaplasia of the respiratory epithelium, a classic response to chronic cigarette smoke exposure. Metaplasia is driven by epigenetic reprogramming (methylation, histone modification, gene silencing) — not mutations — and is therefore potentially reversible if the stimulus is removed. It is an adaptation, not a pre-malignant lesion itself. However, if the noxious stimulus persists, mutations may accumulate, leading to dysplasia → squamous cell carcinoma.

6. A 72-year-old woman is diagnosed with severe aortic stenosis. Echocardiography shows markedly increased left ventricular wall thickness with preserved chamber size. Cardiac catheterization reveals elevated intraventricular pressures. Which of the following best describes the cellular changes occurring in the myocardium, and what is the expected long-term consequence if left untreated?

   1. Hyperplasia of cardiomyocytes; risk of arrhythmia
   2. Hypertrophy of cardiomyocytes; eventual heart failure and fibrosis
   3. Metaplasia of cardiomyocytes to fibroblasts; progressive wall thinning
   4. Atrophy of cardiomyocytes due to pressure overload; ventricular dilation
   5. Dysplasia of cardiomyocytes; progression to cardiac malignancy

   Show Explanation

   Aortic stenosis causes pressure overload, triggering pathologic hypertrophy of cardiomyocytes (increased cell size, not number, as cardiomyocytes are terminally differentiated). Mechanoreceptors sense increased wall stress → activate transcription factors → ↑ protein synthesis. Initially compensatory, chronic pathologic hypertrophy leads to altered architecture, fibrosis, impaired diastolic function, and ultimately heart failure. This contrasts with physiologic hypertrophy (e.g., exercise), which preserves normal architecture and function.

7. A 35-year-old woman is undergoing chemotherapy for breast cancer. Her oncologist explains that the drug works by causing DNA damage in rapidly dividing cells, triggering the intrinsic apoptotic pathway. Which of the following correctly describes the sequence of events in this pathway?

   1. FasL binds Fas → FADD recruitment → Caspase-8 activation → Caspase-3 activation
   2. Perforin pore formation → Granzyme entry → direct caspase activation
   3. Inflammasome activation → Gasdermin pore formation → IL-1β release
   4. Caspase-8 inhibition → RIPK3 activation → membrane rupture
   5. DNA damage → BAX/BAK activation → Cytochrome c release → Apoptosome formation → Caspase-3 activation

   Show Explanation

   The intrinsic (mitochondrial) pathway is triggered by internal stress (DNA damage, ER stress, hormone withdrawal). BAX/BAK permeabilize the mitochondrial outer membrane → cytochrome c release → binds APAF1 + procaspase-9 to form the apoptosome → activates executioner caspase-3. Answer A describes the extrinsic (death receptor) pathway; Answer B describes cytotoxic T-lymphocyte/NK cell killing; Answer D describes pyroptosis; Answer E describes necroptosis.

8. A 52-year-old man is brought to the emergency department with severe epigastric pain that goes through to his back, plus nausea and repeated vomiting for 18 hours. He has drunk about 12 beers a day for 20 years. His temperature is 38.3°C (101°F), pulse 124/min, and blood pressure 88/54 mm Hg. The upper abdomen is tender with guarding, and there is bruising around the belly button. Lab results:

   • Serum lipase: 2,850 U/L
   • Serum calcium: 7.4 mg/dL
   • Serum albumin: 4.0 g/dL

   Despite aggressive fluids, he develops acute respiratory distress syndrome (ARDS) and dies on hospital day 3. A photo of his pancreas and surrounding tissue from the autopsy is shown.

   

   Which of the following processes best explains the chalky white lesions shown in the image?

   1. Protein denaturation from loss of blood supply, with the tissue's outline preserved
   2. Calcium deposits in normal tissue caused by high blood calcium
   3. Release of fatty acids by digestive enzymes, which then bind calcium
   4. Digestion of dead tissue by enzymes from white blood cells, leaving a liquid-filled cavity
   5. A cheese-like, crumbly mass of dead tissue inside a granuloma

   Show Explanation

   In acute pancreatitis, activated pancreatic lipase leaks into the fat around the pancreas and breaks down fat, releasing free fatty acids. These bind calcium and form calcium soaps ("saponification"), which look like chalky white spots on gross exam. On a slide, you'd see pale outlines of dead fat cells with blue-purple, grainy calcium deposits and a rim of inflammation. Because this process uses up calcium, it helps explain the patient's low blood calcium. His albumin is normal, so the low calcium is real, not a lab artifact.