Radiobiology Practice Questions

Q: If radiation exposure occurs before DNA synthesis, how does the damage to the chromosome manifest?

Damage to both arms of the chromosome. **Explanation:** The manifestation of chromosomal damage depends on the timing of radiation exposure relative to the **S-phase (DNA synthesis)** of the cell cycle. **1. Why Option B is Correct:** When radiation exposure occurs **before DNA synthesis (G1 phase)**, the cell contains only a single strand of chromatin. If a break occurs at this stage, it is a "chromosome aberration." During the subsequent S-phase, the damaged DNA template is replicated. Consequently, the break is duplicated, resulting in **damage to both sister chromatids (both arms)** of the chromosome seen during metaphase. **2. Why Other Options are Incorrect:** * **Option A & C:** Damage to only one arm (chromatid aberration) occurs if the radiation exposure happens **after DNA synthesis (G2 phase)**. At this stage, the DNA has already replicated into two sister chromatids. An ionizing hit usually affects only one of the two strands, leaving the other intact. * **Option D:** Radiation is ionizing; it inherently causes single or double-strand breaks in DNA. It is impossible for a significant radiation dose to result in "no arm" being affected if damage has occurred. **High-Yield Clinical Pearls for NEET-PG:** * **Radiosensitivity:** Cells are most sensitive during **Mitosis (M phase)** and late **G2 phase**. They are most resistant during the **S phase**. * **Direct vs. Indirect Action:** Most DNA damage from X-rays is **indirect**, mediated by the radiolysis of water and the production of **hydroxyl (OH•) free radicals**. * **Lethal Aberrations:** Dicentric rings and anaphase bridges are considered lethal chromosomal aberrations. * **Rule of Thumb:** Pre-replication exposure = Chromosome aberration (both arms); Post-replication exposure = Chromatid aberration (one arm).

Q: Which phase of the cell cycle is most sensitive to radiation?

G2 and M. ### Explanation The sensitivity of a cell to ionizing radiation varies significantly across the cell cycle, a concept known as **Radiosensitivity**. **1. Why G2 and M are the most sensitive:** * **M Phase (Mitosis):** This is the **most sensitive** phase. During mitosis, DNA is highly condensed (chromatin condensation), and the cell’s repair mechanisms are least active. Any damage to the DNA during this stage leads to immediate mitotic death or chromosomal aberrations. * **G2 Phase:** This is the second most sensitive phase. The cell is preparing for division, and there is little time for DNA repair before it enters the vulnerable M phase. **2. Why other options are incorrect:** * **S Phase (Synthesis):** This is the **most radioresistant** phase of the cell cycle. During the late S phase, homologous recombination repair mechanisms are most active because the DNA has been replicated, providing a sister chromatid as a template for repair. * **G1 Phase:** Sensitivity in G1 is intermediate. While more sensitive than the S phase, it is significantly less sensitive than the G2/M transition. * **G0 Phase:** These are quiescent cells. While they can be damaged, the "Law of Bergonie and Tribondeau" states that cells with high mitotic activity are more radiosensitive; thus, non-dividing G0 cells are relatively resistant compared to cycling cells. **3. High-Yield Clinical Pearls for NEET-PG:** * **Law of Bergonie and Tribondeau:** Radiosensitivity is directly proportional to the reproductive rate and inversely proportional to the degree of differentiation. * **Most Sensitive Phase:** M phase. * **Most Resistant Phase:** Late S phase. * **Oxygen Enhancement Ratio (OER):** Cells are more sensitive to radiation in the presence of oxygen (Oxygen Effect), which is why hyperbaric oxygen is sometimes used in radiotherapy. * **Order of Sensitivity (Decreasing):** M > G2 > G1 > S.

Q: What is the earliest feature of radiation toxicity?

Erythema. **Explanation:** **1. Why Erythema is Correct:** Erythema is the earliest clinical manifestation of radiation-induced skin injury (radiodermatitis). It typically occurs within hours to days after exposure. The underlying pathophysiology involves **acute vasodilation** and increased capillary permeability caused by the release of inflammatory mediators (like histamine and cytokines) in response to ionizing radiation. This initial "transient erythema" is often followed by a more persistent "main erythema" phase as the basal cell layer of the epidermis begins to fail. **2. Analysis of Incorrect Options:** * **B. Desquamation:** This occurs later than erythema (usually 3–4 weeks post-exposure). It results from the depletion of the basal cell layer, leading to the peeling of the skin. It can be dry (flaking) or moist (serous exudate). * **C. Edematous rough skin:** This is a feature of subacute or chronic radiation damage. Edema occurs due to lymphatic obstruction and chronic inflammation, appearing well after the initial erythematous phase. * **D. Hyperpigmentation:** This is a late-stage sequela of radiation. It occurs weeks to months later due to the stimulation of melanocytes or as a post-inflammatory response following the healing of acute dermatitis. **3. High-Yield Clinical Pearls for NEET-PG:** * **Order of Skin Changes:** Transient Erythema → Main Erythema → Dry Desquamation → Moist Desquamation → Necrosis/Ulceration. * **Radiosensitivity:** The **Basal layer** of the epidermis is the most radiosensitive part of the skin. * **Law of Bergonie and Tribondeau:** Cells that are rapidly dividing, undifferentiated, and have a long mitotic future are the most radiosensitive (e.g., bone marrow, germ cells, intestinal epithelium). * **Threshold Dose:** Acute skin erythema typically occurs at a threshold dose of approximately **2–6 Gy**.

Q: Which of the following tissues is radioresistant?

Cartilage. The radiosensitivity of a tissue is governed by the **Law of Bergonié and Tribondeau**, which states that cells are more radiosensitive if they have a high mitotic rate, a long mitotic future (many future divisions), and are undifferentiated (primitive). ### 1. Why Cartilage is the Correct Answer **Cartilage** is considered **radioresistant** because it consists of highly differentiated cells (chondrocytes) that have a very low mitotic rate and poor vascularity. Since the cells do not divide frequently, radiation-induced DNA damage does not manifest as immediate cell death. Other radioresistant tissues include mature bone, muscle, and nerve cells. ### 2. Analysis of Incorrect Options * **Seminoma (Option B):** This is a germ cell tumor and is one of the most **radiosensitive** tumors in the human body. Germ cells are primitive and rapidly dividing. * **Ewing’s Sarcoma (Option C):** This is a highly malignant small round blue cell tumor. Unlike mature bone or cartilage, these cells are undifferentiated and rapidly proliferating, making them **radiosensitive**. * **GI Epithelium (Option D):** The lining of the gastrointestinal tract is a **radiosensitive** tissue because the stem cells in the crypts are constantly dividing to replace the mucosal surface. ### 3. High-Yield Clinical Pearls for NEET-PG * **Most Radiosensitive Phase of Cell Cycle:** M phase (Mitosis), followed by G2. * **Most Radioresistant Phase:** Late S phase (due to DNA repair mechanisms). * **Most Radiosensitive Cell:** Lymphocyte (Exception to the Law of Bergonié and Tribondeau, as it is a mature cell but highly sensitive). * **Order of Sensitivity (High to Low):** Lymphocytes > Gonads > GI Epithelium > Skin > Endothelium > Growing Bone/Cartilage > Muscle > Nerve.

Q: Which of the following is NOT a radiosensitizing drug?

Cyclophosphamide. ### Explanation **Radiosensitizers** are drugs that enhance the lethal effects of ionizing radiation on tumor cells, often by interfering with DNA repair or synchronizing cells into a sensitive phase of the cell cycle. **1. Why Cyclophosphamide is the correct answer:** Cyclophosphamide is a potent **alkylating agent** used primarily as a cytotoxic chemotherapy drug. While it kills cancer cells by cross-linking DNA, it is **not** classified as a radiosensitizer. In fact, combining cyclophosphamide with radiation can significantly increase systemic toxicity and local tissue damage (like hemorrhagic cystitis or lung fibrosis) without specifically sensitizing the tumor to the radiation dose. **2. Analysis of Incorrect Options (Radiosensitizers):** * **5-Fluorouracil (5-Fu):** A classic radiosensitizer. It inhibits thymidylate synthase, depleting the pool of nucleotides required for DNA repair after radiation damage. It is commonly used in gastrointestinal and head/neck cancers. * **Bromodeoxyuridine (BUDR):** A halogenated pyrimidine that incorporates into DNA in place of thymidine. This makes the DNA strand more susceptible to breakage when exposed to radiation. * **Hydroxyurea:** This drug inhibits ribonucleotide reductase and arrests cells in the **G1-S phase** boundary. Since cells are most radiosensitive in the G2 and M phases, and relatively sensitive in late G1/early S, hydroxyurea helps synchronize the cell population for more effective killing. ### NEET-PG High-Yield Pearls: * **Most Radiosensitive Phase:** M phase (followed by G2). * **Most Radioresistant Phase:** Late S phase. * **Hypoxic Cell Sensitizers:** Misonidazole and Nimorazole (mimic oxygen to fix radiation damage). * **Radioprotectors:** Amifostine (scavenges free radicals to protect normal tissue). * **Common Radiosensitizers (Mnemonic: "5-BHC"):** **5**-FU, **B**UDR, **H**ydroxyurea, **C**isplatin.

Question 1

If radiation exposure occurs before DNA synthesis, how does the damage to the chromosome manifest?

Accepted Answer

Damage to both arms of the chromosome

Answer

Damage to one arm of the chromosome

Answer

Damage to the shorter arm of the chromosome

Answer

No arm of the chromosome is affected

Question 2

Which phase of the cell cycle is most sensitive to radiation?

Accepted Answer

G2 and M

Answer

G1 and S

Answer

G1 and G2

Answer

G0 and G1

Question 3

Diagnostic radiation in a pregnant woman may cause which of the following?

Accepted Answer

Low birth weight babies

Answer

High fetal exposure

Answer

Abortion

Answer

Genetic abnormalities in the fetus

Question 4

Which of the following tumors is not radiosensitive?

Accepted Answer

Melanoma

Answer

Lymphoma

Answer

Seminoma

Answer

Multiple myeloma

Question 5

Which type of cells are most radiosensitive?

Accepted Answer

Vegetative intermitotic cells

Answer

Differential intermitotic cells

Answer

Reverting post mitotic cells

Answer

Fixed post mitotic cells

Question 6

Which of the following is the most radiosensitive?

Accepted Answer

Carcinoma of the ovary

Answer

Carcinoma of the cervix

Answer

Carcinoma of the kidney

Answer

Carcinoma of the pancreas

Question 7

What is the earliest feature of radiation toxicity?

Accepted Answer

Erythema

Answer

Desquamation

Answer

Edematous rough skin

Answer

Hyperpigmentation

Question 8

Which of the following tissues is radioresistant?

Accepted Answer

Cartilage

Answer

Seminoma

Answer

Ewing's sarcoma

Answer

GI epithelium

Question 9

Which phase of the cell cycle is most radioresistant?

Accepted Answer

S Phase

Answer

G1 Phase

Answer

G2 Phase

Answer

M phase

Question 10

Which of the following is NOT a radiosensitizing drug?

Accepted Answer

Cyclophosphamide

Answer

5-Fluorouracil (5-Fu)

Answer

Bromodeoxyuridine (BUDR)

Answer

Hydroxyurea

Radiobiology — MCQs

Radiobiology — MCQs

On this page

Practice by Chapter

Want unlimited practice?