Pathophysiology Quiz: Neoplasia Basics
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Neoplasia BasicsQuestion 1 of 20

The final and most inefficient step of the metastatic cascade is colonization, where a disseminated tumor cell gives rise to a new tumor. What is a major challenge that the tumor cell must overcome specifically during this colonization phase, after it has already extravasated into a new tissue?

Surviving the shear stress and anoikis signals present in the circulatory system.
Undergoing an epithelial-mesenchymal transition to acquire motility.
Breaching the vascular endothelium to exit the bloodstream.
Adapting its metabolism and survival signaling to a foreign tissue microenvironment.
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Pathophysiology Quiz

Pathophysiology Quiz: Neoplasia Basics

Practice Neoplasia Basics in Pathophysiology with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Neoplasia Basics, giving you a quick way to practice the rules, question types, and explanations that matter most for Pathophysiology.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

The final and most inefficient step of the metastatic cascade is colonization, where a disseminated tumor cell gives rise to a new tumor. What is a major challenge that the tumor cell must overcome specifically during this colonization phase, after it has already extravasated into a new tissue?

  1. Surviving the shear stress and anoikis signals present in the circulatory system.
  2. Undergoing an epithelial-mesenchymal transition to acquire motility.
  3. Breaching the vascular endothelium to exit the bloodstream.
  4. Adapting its metabolism and survival signaling to a foreign tissue microenvironment. (correct answer)
Explanation: When analyzing metastatic cascade questions, focus on the sequential nature of each step and what specific challenges arise at each phase. The metastatic cascade follows a precise order: local invasion → intravasation → circulation survival → extravasation → colonization. Colonization is indeed the final and most inefficient step because disseminated tumor cells must establish themselves in a completely foreign microenvironment. After successfully extravasating, the tumor cell faces the enormous challenge of adapting to a tissue with different growth factors, extracellular matrix composition, oxygen levels, nutrient availability, and cellular signaling networks than its tissue of origin. Most disseminated cells die because they cannot reprogram their metabolism and survival pathways to thrive in this new environment. This explains why colonization has such low efficiency rates. Option A describes challenges during circulation, not colonization. Shear stress and anoikis (detachment-induced cell death) occur while tumor cells are still in the bloodstream, before they've extravasated. Option B refers to epithelial-mesenchymal transition (EMT), which occurs much earlier during local invasion and intravasation when cells need to acquire motility and invasive properties. Option C describes extravasation itself—the process of breaching the endothelium to exit circulation. This happens immediately before colonization, not during it. Remember this sequence: colonization begins after extravasation is complete. The key challenge isn't getting out of vessels or surviving circulation—it's adapting to survive and proliferate in an entirely different tissue environment with foreign molecular cues.

Question 2

A cancer cell acquires a mutation that allows it to survive and proliferate in the absence of attachment to an extracellular matrix. This characteristic is a crucial prerequisite for metastasis because it allows the cell to overcome which specific challenge?

  1. Contact inhibition upon reaching confluence in the primary tumor.
  2. Hypoxia in the center of a poorly vascularized tumor.
  3. Immune surveillance by natural killer (NK) cells.
  4. Anoikis during transit in the bloodstream and lymphatics. (correct answer)
Explanation: When you encounter questions about metastasis prerequisites, focus on the specific cellular barriers cancer cells must overcome at each step of the metastatic cascade. The ability to survive without matrix attachment directly addresses a critical survival mechanism. Normal cells undergo anoikis — programmed cell death triggered when they detach from their extracellular matrix. This protective mechanism prevents displaced cells from surviving in inappropriate locations. For cancer cells to metastasize successfully, they must first acquire resistance to anoikis, allowing them to survive as they detach from the primary tumor, travel through circulation, and eventually seed distant sites. Answer D is correct because overcoming anoikis is essential for survival during the transit phase of metastasis. Without this adaptation, detached cancer cells would simply die in the bloodstream or lymphatics before reaching potential metastatic sites. Answer A is wrong because contact inhibition affects growth rate but doesn't prevent survival during detachment and transit. Answer B is wrong because hypoxia resistance helps cells survive in poorly vascularized areas but isn't specifically related to matrix-independent survival during metastasis. Answer C is wrong because NK cell evasion involves different mechanisms (like altered MHC expression) rather than matrix attachment independence. Study tip: Remember that metastasis requires sequential adaptations. Anoikis resistance is the foundational requirement — without it, cancer cells can't survive the journey to become metastatic. Focus on understanding the step-by-step barriers in the metastatic cascade when studying cancer pathophysiology.

Question 3

Which statement best distinguishes the genetic basis of oncogene activation from that of tumor suppressor gene (TSG) inactivation?

  1. Oncogenic mutations are typically dominant at the cellular level, requiring only one mutated allele, while TSG inactivation is typically recessive. (correct answer)
  2. Oncogenes arise from proto-oncogenes via loss-of-function mutations, whereas TSGs are inactivated by gain-of-function mutations.
  3. Activation of an oncogene requires epigenetic silencing, while inactivation of a TSG requires a chromosomal translocation.
  4. Oncogenes are exclusively viral in origin, whereas TSGs are host-derived genes that regulate the cell cycle.
Explanation: When you encounter questions about cancer genetics, focus on the fundamental difference in how oncogenes and tumor suppressor genes (TSGs) behave at the cellular level—it's all about dominance patterns. Option A is correct because it captures this key distinction. Oncogenes act dominantly: when a proto-oncogene mutates into an oncogene, just one "bad" copy can drive the cell toward cancer. Think of it like a stuck accelerator pedal—you only need one pedal to malfunction to lose control. In contrast, TSGs follow Knudson's "two-hit hypothesis"—they're recessive, meaning you typically need both alleles inactivated before losing tumor suppression. It's like having two backup brakes: losing one isn't catastrophic, but losing both is dangerous. Option B reverses the mutation types. Oncogenes arise from gain-of-function mutations that make proto-oncogenes hyperactive, while TSGs are lost through loss-of-function mutations that eliminate their protective effects. Option C incorrectly specifies mechanisms. While epigenetic changes and translocations can affect these genes, they're not the defining or required mechanisms. Both gene types can be altered through various mechanisms including point mutations, deletions, and amplifications. Option D is factually wrong. Oncogenes aren't exclusively viral—most are mutated versions of normal cellular proto-oncogenes. Plus, both oncogenes and TSGs can be found in viruses and host cells. Study tip: Remember "dominant oncogenes, recessive TSGs" as your foundational rule. This dominance pattern explains why oncogenes are often associated with translocations (activating one copy) while TSGs are linked with deletions (removing both copies).

Question 4

A biopsy of a colon tumor reveals that the MLH1 gene, a key component of the DNA mismatch repair system, is not being expressed. However, DNA sequencing shows a wild-type coding sequence for MLH1. Which of the following is the most likely mechanism for the inactivation of this tumor suppressor gene in this tumor?

  1. A chromosomal translocation that fuses MLH1 to an actively transcribed gene.
  2. Promoter hypermethylation leading to transcriptional silencing. (correct answer)
  3. A point mutation in a microRNA that normally degrades MLH1 mRNA.
  4. Constitutive phosphorylation of the MLH1 protein, leading to its degradation.
Explanation: When a tumor suppressor gene is inactivated without a change in its DNA sequence, an epigenetic mechanism is often responsible. The most common of these is the hypermethylation of CpG islands located in the promoter region of the gene. This methylation pattern recruits proteins that condense chromatin, making the promoter inaccessible to transcription factors and RNA polymerase, thereby silencing gene expression.

Question 5

For a carcinoma cell to metastasize, it must first detach from the primary tumor and invade surrounding tissue. This process often involves epithelial-mesenchymal transition (EMT). Which pair of molecular changes is most characteristic of a cell undergoing EMT?

  1. Upregulation of E-cadherin and downregulation of vimentin.
  2. Downregulation of E-cadherin and upregulation of N-cadherin. (correct answer)
  3. Activation of caspases and fragmentation of nuclear DNA.
  4. Increased expression of p53 and induction of cell cycle arrest.
Explanation: EMT involves a phenotypic switch from a stationary, polarized epithelial cell to a migratory, spindle-shaped mesenchymal cell. A key molecular event is the loss of the cell-cell adhesion molecule E-cadherin, which allows the cell to detach. Concurrently, there is often a 'cadherin switch' to N-cadherin and an upregulation of mesenchymal markers like vimentin, which are associated with increased motility and invasion.

Question 6

Disseminated tumor cells may remain dormant for many years at a secondary site before initiating outgrowth. Which of the following is a key proposed mechanism for maintaining this state of cellular dormancy?

  1. A failure to induce sufficient angiogenesis, limiting the tumor mass to a microscopic size. (correct answer)
  2. A rapid acquisition of additional driver mutations that confer a highly aggressive phenotype.
  3. Complete downregulation of all MHC class I molecules to ensure perfect immune evasion.
  4. A permanent exit from the cell cycle, leading to a state of irreversible senescence.
Explanation: When you encounter questions about tumor dormancy, focus on the biological constraints that prevent microscopic metastatic cells from growing into clinically detectable tumors. This is a critical concept in cancer biology that explains why some patients develop metastases years or decades after their primary tumor was removed. The key mechanism maintaining cellular dormancy is angiogenic failure (option A). Disseminated tumor cells can survive at secondary sites, but without establishing their own blood supply, they cannot grow beyond 1-2 millimeters in diameter. This creates a balance between cell proliferation and cell death, maintaining the tumor mass at a microscopic, dormant state. When these cells eventually acquire the ability to stimulate angiogenesis—often called the "angiogenic switch"—they can break dormancy and begin rapid outgrowth. Option B is incorrect because acquiring aggressive driver mutations would actually break dormancy, not maintain it. These mutations typically promote growth and proliferation. Option C misunderstands immune evasion—while MHC downregulation can help tumors avoid immune detection, complete downregulation isn't necessary for dormancy, and "perfect" immune evasion doesn't exist. Option D confuses dormancy with senescence. Dormant cells retain the potential to resume proliferation (which is why they're dangerous), whereas senescent cells have permanently exited the cell cycle. Study tip: Remember that tumor dormancy is a reversible state of growth arrest, primarily limited by angiogenesis. This distinguishes it from permanent states like senescence and explains why metastases can appear years after apparent cancer cure.

Question 7

The MYC proto-oncogene is frequently activated in human cancers. In Burkitt's lymphoma, a t(8;14) chromosomal translocation is the characteristic mechanism of activation. What is the direct functional consequence of this specific translocation?

  1. It creates a MYC-IgH fusion protein with enhanced transcriptional activity.
  2. It causes multiple point mutations within the MYC coding sequence, making the protein hyperstable.
  3. It places the MYC gene under the regulatory control of the immunoglobulin heavy chain locus. (correct answer)
  4. It deletes the entire MYC gene, leading to a loss of cell cycle regulation and increased proliferation.
Explanation: In Burkitt's lymphoma, the translocation moves the MYC gene from its normal location on chromosome 8 to chromosome 14, placing it adjacent to the immunoglobulin heavy chain (IgH) gene locus. The IgH locus is highly and constitutively active in B lymphocytes. As a result, MYC is now driven by the powerful IgH enhancer elements, leading to its massive and inappropriate overexpression. This is a classic example of oncogene activation by promoter/enhancer substitution, not by creating a fusion protein or point mutation.

Question 8

A woman with a germline mutation in the BRCA1 gene has a significantly increased lifetime risk of breast and ovarian cancer. The tumor suppressor function of the wild-type BRCA1 protein is most directly related to its role in which fundamental cellular process?

  1. Regulation of the G1/S checkpoint by binding to and inactivating E2F.
  2. Induction of apoptosis in response to oncogenic stress by activating BAX.
  3. Negative regulation of the Wnt signaling pathway by promoting β-catenin degradation.
  4. Maintenance of genomic stability through repair of double-strand DNA breaks. (correct answer)
Explanation: When you encounter questions about tumor suppressor genes like BRCA1, focus on understanding their specific cellular functions rather than memorizing general cancer pathways. Tumor suppressors each have distinct roles in preventing malignant transformation. BRCA1 (Breast Cancer 1) functions primarily as a DNA damage response protein. The wild-type BRCA1 protein is essential for homologous recombination repair, the cell's most accurate mechanism for fixing double-strand DNA breaks. When DNA damage occurs, BRCA1 helps recruit and coordinate repair proteins at the damage site, ensuring genomic stability. Without functional BRCA1, cells accumulate DNA damage over time, leading to the chromosomal instability characteristic of BRCA1-associated cancers. This explains why BRCA1 mutations dramatically increase cancer risk—the cell loses a critical DNA repair mechanism. Looking at the wrong answers: Choice A describes the retinoblastoma (Rb) protein's function in cell cycle control, not BRCA1. Choice B refers to p53's role in apoptosis induction—while p53 and BRCA1 can interact in DNA damage responses, this isn't BRCA1's primary tumor suppressor function. Choice C describes APC's role in the Wnt pathway, which is unrelated to BRCA1's mechanism. Remember that different tumor suppressors have specialized functions: p53 is the "guardian of the genome" for damage detection and apoptosis, Rb controls the G1/S checkpoint, APC regulates cell adhesion and Wnt signaling, and BRCA1/BRCA2 specifically maintain genomic stability through DNA repair. Matching the gene to its specific cellular role is key to answering these questions correctly.

Question 9

The Philadelphia chromosome, characteristic of chronic myeloid leukemia (CML), results from a translocation between chromosomes 9 and 22. This creates the BCR-ABL fusion gene. Which of the following best describes the oncogenic mechanism of the resulting BCR-ABL protein?

  1. It functions as a transcription factor that inappropriately activates genes involved in angiogenesis and cell motility.
  2. It binds to and sequesters the p53 tumor suppressor protein, preventing the cell from undergoing apoptosis.
  3. It possesses constitutive tyrosine kinase activity that continuously activates pro-proliferation and pro-survival signaling pathways. (correct answer)
  4. It disrupts the function of DNA mismatch repair machinery, leading to a rapid accumulation of mutations throughout the genome.
Explanation: The ABL gene codes for a tightly regulated tyrosine kinase. The fusion with BCR leads to dimerization and constitutive (continuous, signal-independent) activation of this kinase. The BCR-ABL protein then phosphorylates numerous downstream substrates, leading to uncontrolled proliferation of granulocyte precursors, which is the hallmark of CML. The other options describe mechanisms of different oncoproteins or tumor suppressor inactivation.

Question 10

A malignant melanoma cell line is found to harbor two specific mutations: a gain-of-function mutation in the BRAF gene (V600E) and a loss-of-function mutation in the PTEN gene. What is the most likely combined effect of these two alterations on intracellular signaling?

  1. Synergistic and constitutive activation of both the MAPK and PI3K/AKT pathways. (correct answer)
  2. Complete blockage of the G1/S checkpoint due to hyperphosphorylation of the Rb protein.
  3. Activation of the MAPK pathway but simultaneous inhibition of the PI3K/AKT pathway.
  4. A paradoxical induction of apoptosis due to conflicting intracellular growth signals.
Explanation: BRAF is a key component of the RAS-RAF-MEK-ERK (MAPK) signaling cascade. The V600E mutation makes it constitutively active. PTEN is a tumor suppressor phosphatase that is the major negative regulator of the PI3K/AKT pathway. Losing PTEN function leads to constitutive activation of AKT. Therefore, having both mutations results in the simultaneous, potent, and synergistic activation of two major pro-growth and pro-survival pathways.

Question 11

A patient with metastatic colon cancer is treated with bevacizumab, a monoclonal antibody that targets Vascular Endothelial Growth Factor (VEGF). This therapy is primarily intended to disrupt which step of tumor progression?

  1. The initial epithelial-mesenchymal transition of cells at the primary tumor site.
  2. The survival of circulating tumor cells in the bloodstream and lymphatic system.
  3. The outgrowth of micrometastases into macroscopic tumors in distant organs. (correct answer)
  4. The degradation of the basement membrane by matrix metalloproteinases.
Explanation: VEGF is the primary signaling molecule that stimulates angiogenesis (the formation of new blood vessels). While angiogenesis is important for primary tumor growth, it is also critical for the growth of metastases. Micrometastases cannot grow beyond a few millimeters in diameter without inducing their own blood supply. By inhibiting VEGF, bevacizumab blocks this process, starving the micrometastases of oxygen and nutrients and thereby preventing their colonization and outgrowth into clinically significant tumors.

Question 12

A 45-year-old male with a strong family history of early-onset colorectal cancer is found to carry a germline mutation in one allele of the APC gene. According to the multi-step model of colorectal carcinogenesis, which subsequent molecular event is most critical for the initiation of an adenomatous polyp in his colonic epithelial cells?

  1. A gain-of-function mutation in a K-RAS proto-oncogene.
  2. A somatic mutation or loss of the second, wild-type APC allele. (correct answer)
  3. A second, independent germline mutation affecting the remaining functional APC allele.
  4. Epigenetic silencing of a growth-promoting gene, such as MYC.
Explanation: The APC gene is a tumor suppressor that requires inactivation of both alleles for its function to be lost (Knudson's 'two-hit' hypothesis). In familial adenomatous polyposis (FAP), an individual inherits one non-functional (mutated) allele. The critical initiating event for polyp formation is the 'second hit'—a somatic inactivation of the remaining wild-type allele in a colon cell. This leads to loss of APC function, dysregulation of β-catenin, and uncontrolled proliferation.

Question 13

A glioblastoma cell line is analyzed and found to have lost one copy of chromosome 10, a region containing the PTEN tumor suppressor gene. The remaining PTEN allele is unmutated but produces no protein. Which mechanism most likely accounts for the inactivation of the second allele?

  1. Gene amplification.
  2. Activating point mutation.
  3. Epigenetic silencing via promoter methylation. (correct answer)
  4. Fusion with an actively transcribed gene via translocation.
Explanation: This scenario describes a classic 'two-hit' inactivation of a tumor suppressor gene. The first 'hit' is the physical loss of one allele through chromosomal deletion. The second 'hit' must inactivate the remaining allele. Since the question states the second allele is unmutated, epigenetic silencing through hypermethylation of its promoter is the most plausible mechanism to prevent its transcription and protein production. Gene amplification (A) and activating mutations (B) apply to oncogenes, not TSG inactivation. Fusion (D) is a less common mechanism for TSG silencing compared to methylation.

Question 14

A biopsy of a colon tumor reveals that the MLH1 gene, a key component of the DNA mismatch repair system, is not being expressed. However, DNA sequencing shows a wild-type coding sequence for MLH1. Which of the following is the most likely mechanism for the inactivation of this tumor suppressor gene in this tumor?

  1. A chromosomal translocation that fuses MLH1 to an actively transcribed gene.
  2. Promoter hypermethylation leading to transcriptional silencing. (correct answer)
  3. A point mutation in a microRNA that normally degrades MLH1 mRNA.
  4. Constitutive phosphorylation of the MLH1 protein, leading to its degradation.
Explanation: When a tumor suppressor gene is inactivated without a change in its DNA sequence, an epigenetic mechanism is often responsible. The most common of these is the hypermethylation of CpG islands located in the promoter region of the gene. This methylation pattern recruits proteins that condense chromatin, making the promoter inaccessible to transcription factors and RNA polymerase, thereby silencing gene expression.

Question 15

A breast cancer metastasizes preferentially to bone rather than to the spleen or kidney. This phenomenon of organotropism is best explained by which of the following concepts?

  1. The anatomical circulatory pattern, which dictates that bone is the first capillary bed encountered by disseminating tumor cells.
  2. The 'seed and soil' hypothesis, where the bone microenvironment provides specific factors that support tumor cell survival. (correct answer)
  3. The higher rate of proliferation of tumor cells that are arrested in the narrow sinusoids of the bone marrow.
  4. The absence of a resident immune cell population within bone tissue, allowing tumor cells to escape surveillance.
Explanation: The 'seed and soil' hypothesis, proposed by Stephen Paget, posits that metastasis depends on the interaction between the disseminated tumor cell (the 'seed') and the microenvironment of the distant organ (the 'soil'). The bone microenvironment is rich in growth factors, chemokines, and extracellular matrix components that are uniquely supportive for the survival, dormancy, and eventual outgrowth of certain breast cancer cells, explaining this organ-specific metastasis.

Question 16

A malignant melanoma cell line is found to harbor two specific mutations: a gain-of-function mutation in the BRAF gene (V600E) and a loss-of-function mutation in the PTEN gene. What is the most likely combined effect of these two alterations on intracellular signaling?

  1. Synergistic and constitutive activation of both the MAPK and PI3K/AKT pathways. (correct answer)
  2. Complete blockage of the G1/S checkpoint due to hyperphosphorylation of the Rb protein.
  3. Activation of the MAPK pathway but simultaneous inhibition of the PI3K/AKT pathway.
  4. A paradoxical induction of apoptosis due to conflicting intracellular growth signals.
Explanation: BRAF is a key component of the RAS-RAF-MEK-ERK (MAPK) signaling cascade. The V600E mutation makes it constitutively active. PTEN is a tumor suppressor phosphatase that is the major negative regulator of the PI3K/AKT pathway. Losing PTEN function leads to constitutive activation of AKT. Therefore, having both mutations results in the simultaneous, potent, and synergistic activation of two major pro-growth and pro-survival pathways.

Question 17

The retinoblastoma protein (Rb) is a tumor suppressor that controls the G1/S checkpoint. Its function is regulated by phosphorylation. Which of the following events would have a functional consequence most similar to a biallelic loss-of-function mutation in the RB1 gene?

  1. A mutation in the E2F transcription factor that prevents it from binding to DNA.
  2. Increased expression of the cyclin-dependent kinase inhibitor p16 (CDKN2A).
  3. A chromosomal amplification leading to overexpression of the Cyclin D1 gene. (correct answer)
  4. Stabilization and activation of the p53 protein in response to DNA damage.
Explanation: The Rb protein's function is to bind and sequester the E2F transcription factor, preventing the expression of genes required for S-phase. Rb is inactivated when it is hyperphosphorylated by Cyclin D/CDK4/6 complexes. Overexpression of Cyclin D leads to constitutive hyperphosphorylation and inactivation of Rb, thus releasing E2F. This functionally mimics the loss of the Rb protein itself, as in both cases E2F is constitutively active.

Question 18

A woman with a germline mutation in the BRCA1 gene has a significantly increased lifetime risk of breast and ovarian cancer. The tumor suppressor function of the wild-type BRCA1 protein is most directly related to its role in which fundamental cellular process?

  1. Regulation of the G1/S checkpoint by binding to and inactivating E2F.
  2. Induction of apoptosis in response to oncogenic stress by activating BAX.
  3. Negative regulation of the Wnt signaling pathway by promoting β-catenin degradation.
  4. Maintenance of genomic stability through repair of double-strand DNA breaks. (correct answer)
Explanation: When you encounter questions about tumor suppressor genes like BRCA1, focus on understanding their specific cellular functions rather than memorizing general cancer pathways. Tumor suppressors each have distinct roles in preventing malignant transformation. BRCA1 (Breast Cancer 1) functions primarily as a DNA damage response protein. The wild-type BRCA1 protein is essential for homologous recombination repair, the cell's most accurate mechanism for fixing double-strand DNA breaks. When DNA damage occurs, BRCA1 helps recruit and coordinate repair proteins at the damage site, ensuring genomic stability. Without functional BRCA1, cells accumulate DNA damage over time, leading to the chromosomal instability characteristic of BRCA1-associated cancers. This explains why BRCA1 mutations dramatically increase cancer risk—the cell loses a critical DNA repair mechanism. Looking at the wrong answers: Choice A describes the retinoblastoma (Rb) protein's function in cell cycle control, not BRCA1. Choice B refers to p53's role in apoptosis induction—while p53 and BRCA1 can interact in DNA damage responses, this isn't BRCA1's primary tumor suppressor function. Choice C describes APC's role in the Wnt pathway, which is unrelated to BRCA1's mechanism. Remember that different tumor suppressors have specialized functions: p53 is the "guardian of the genome" for damage detection and apoptosis, Rb controls the G1/S checkpoint, APC regulates cell adhesion and Wnt signaling, and BRCA1/BRCA2 specifically maintain genomic stability through DNA repair. Matching the gene to its specific cellular role is key to answering these questions correctly.

Question 19

A patient with metastatic colon cancer is treated with bevacizumab, a monoclonal antibody that targets Vascular Endothelial Growth Factor (VEGF). This therapy is primarily intended to disrupt which step of tumor progression?

  1. The initial epithelial-mesenchymal transition of cells at the primary tumor site.
  2. The survival of circulating tumor cells in the bloodstream and lymphatic system.
  3. The outgrowth of micrometastases into macroscopic tumors in distant organs. (correct answer)
  4. The degradation of the basement membrane by matrix metalloproteinases.
Explanation: VEGF is the primary signaling molecule that stimulates angiogenesis (the formation of new blood vessels). While angiogenesis is important for primary tumor growth, it is also critical for the growth of metastases. Micrometastases cannot grow beyond a few millimeters in diameter without inducing their own blood supply. By inhibiting VEGF, bevacizumab blocks this process, starving the micrometastases of oxygen and nutrients and thereby preventing their colonization and outgrowth into clinically significant tumors.

Question 20

A glioblastoma cell line is analyzed and found to have lost one copy of chromosome 10, a region containing the PTEN tumor suppressor gene. The remaining PTEN allele is unmutated but produces no protein. Which mechanism most likely accounts for the inactivation of the second allele?

  1. Gene amplification.
  2. Activating point mutation.
  3. Epigenetic silencing via promoter methylation. (correct answer)
  4. Fusion with an actively transcribed gene via translocation.
Explanation: This scenario describes a classic 'two-hit' inactivation of a tumor suppressor gene. The first 'hit' is the physical loss of one allele through chromosomal deletion. The second 'hit' must inactivate the remaining allele. Since the question states the second allele is unmutated, epigenetic silencing through hypermethylation of its promoter is the most plausible mechanism to prevent its transcription and protein production. Gene amplification (A) and activating mutations (B) apply to oncogenes, not TSG inactivation. Fusion (D) is a less common mechanism for TSG silencing compared to methylation.