Prenatal Diagnosis and Genetic Counseling — MCQs
All of the following are true about Down syndrome except for one.
Disputed maternity can be solved by using the following tests, EXCEPT:
Which of the following markers is not used in quadruple test for antenatal detection of Down syndrome?
Which of the following is the MOST accurate test for detecting neural tube defects?
Number of chromosomes in Klinefelter syndrome:
Nuchal translucency is used in
Most appropriate time for chorionic villus sampling in pregnancy is:
Most commonly used sonological indicator for aneuploidy in first trimester is:
18 weeks pregnant female presents with no high risk of NTD and low risk of trisomy 21 on quad test. What is the most appropriate next step in management?
Which of the following statements about chorionic villus sampling is false?
Prenatal Diagnosis and Genetic Counseling Indian Medical PG Practice Questions and MCQs
Question 1: All of the following are true about Down syndrome except for one.
- A. Incidence of Robertsonian translocation is 1:1000 (Correct Answer)
- B. Most common cause is trisomy 21
- C. Mosaicism 21 has no association with maternal age
- D. Extra chromosome is of maternal origin
- E. Incidence increases with advanced maternal age
Explanation: **Incidence of Robertsonian translocation is 1:1000** - This statement is **not accurate** for Down syndrome. Robertsonian translocation accounts for only about **3-4% of Down syndrome cases**, not a general population incidence of 1:1000. - The vast majority of Down syndrome cases (~95%) are due to trisomy 21 from nondisjunction, not translocation. - This is the **correct answer** as it is the FALSE statement. *Extra chromosome is of maternal origin* - In approximately **90-95% of Down syndrome cases**, the extra copy of chromosome 21 originates from the mother due to **nondisjunction** during meiosis. - This maternal origin is strongly correlated with **advanced maternal age**. *Most common cause is trisomy 21* - **Trisomy 21** (due to meiotic nondisjunction) accounts for about **95% of all Down syndrome cases**, making it the most common genetic mechanism. - This results in three separate copies of chromosome 21 in all body cells. *Mosaicism 21 has no association with maternal age* - **Mosaic Down syndrome** occurs when nondisjunction happens *after fertilization* in early embryonic development, leading to a mixture of cells with normal and trisomic cells. - Because it is a **post-zygotic event**, its incidence is independent of **maternal age**, unlike full trisomy 21. *Incidence increases with advanced maternal age* - **TRUE statement** - The risk of Down syndrome (particularly trisomy 21) increases significantly with **maternal age**. - Risk is approximately 1:1500 at age 20, 1:1000 at age 30, 1:400 at age 35, and 1:100 at age 40. - This is due to increased risk of meiotic nondisjunction in older oocytes.
Question 2: Disputed maternity can be solved by using the following tests, EXCEPT:
- A. Blood grouping
- B. HLA typing
- C. DNA fingerprinting
- D. Precipitin test (Correct Answer)
Explanation: ***Precipitin test*** - The **precipitin test** is used to determine the origin of a **blood sample**, specifically whether it is **human or animal blood**, by detecting species-specific proteins. It is not used for assessing maternity. - This test is primarily employed in **forensic serology** to differentiate between blood from different animal species, making it irrelevant for paternity or maternity disputes. *Blood grouping* - **Blood grouping** (e.g., ABO and Rh systems) can be used to **exclude paternity or maternity** by comparing the blood types of the child, mother, and alleged father. - If the child's blood type is incompatible with the alleged parents based on Mendelian inheritance, one or both can be excluded. *HLA typing* - **HLA typing** (Human Leukocyte Antigen) is a more powerful genetic marker system than ABO/Rh for determining paternity or maternity. - It involves analyzing highly polymorphic genes on chromosome 6 that encode cell surface proteins, providing a more definitive means of **inclusion or exclusion**. *DNA fingerprinting* - **DNA fingerprinting** (also known as **DNA profiling**) is the **most accurate and widely accepted method** for resolving paternity and maternity disputes. - It analyzes highly variable regions of DNA unique to each individual, providing a statistically strong basis for **inclusion or exclusion** by comparing genetic profiles.
Question 3: Which of the following markers is not used in quadruple test for antenatal detection of Down syndrome?
- A. Inhibin
- B. Estradiol (Correct Answer)
- C. AFP
- D. ss-hCG
Explanation: ***Estradiol*** - **Estradiol** is a primary **estrogen** produced by the ovaries and placenta but is **not** one of the four markers included in the **quadruple screen** for Down syndrome. - The quadruple screen typically measures levels of **alpha-fetoprotein (AFP)**, **unconjugated estriol (uE3)**, **human chorionic gonadotropin (hCG)**, and **inhibin A**. *Inhibin* - **Inhibin A** is one of the four components of the **quadruple screen** for Down syndrome. - In pregnancies affected by Down syndrome, inhibin A levels are typically **elevated**. *AFP* - **Alpha-fetoprotein (AFP)** is a key component of the **quadruple screen**. - In a Down syndrome pregnancy, maternal serum AFP levels are typically **lower** than normal. *ss-hCG* - **Beta-human chorionic gonadotropin (β-hCG)** is a specific subunit of hCG and is one of the four markers in the **quadruple screen**. - In pregnancies with Down syndrome, maternal serum β-hCG levels are usually **elevated**.
Question 4: Which of the following is the MOST accurate test for detecting neural tube defects?
- A. USG (Correct Answer)
- B. Placentography
- C. Chromosomal analysis
- D. Amniocentesis
Explanation: ***USG (Ultrasound)*** - **High-resolution ultrasound** is the **gold standard and most accurate imaging modality** for detecting **neural tube defects (NTDs)** due to its ability to directly visualize anatomical structures of the fetus. - **Diagnostic accuracy**: Detection rate >95% for anencephaly and 80-90% for open spina bifida with targeted anomaly scan at 18-20 weeks. - Can identify specific features such as **lemon sign** (frontal bone scalloping), **banana sign** (cerebellar compression), direct visualization of **spina bifida**, **anencephaly**, or **encephalocele**. - **Non-invasive, safe, and widely available**, making it the primary diagnostic tool in clinical practice. *Amniocentesis* - **Amniocentesis** measures **alpha-fetoprotein (AFP)** and **acetylcholinesterase (AChE)** in amniotic fluid, which are elevated in open NTDs. - While highly accurate as a **confirmatory test** (near 99% sensitivity with both markers), it is **invasive** with risk of miscarriage (0.1-0.3%). - Used primarily when ultrasound findings are **equivocal** or for **biochemical confirmation**, not as the first-line diagnostic test. - In modern practice, ultrasound has largely replaced amniocentesis for NTD diagnosis due to superior imaging technology. *Chromosomal analysis* - **Chromosomal analysis** (karyotyping) detects **chromosomal abnormalities** like trisomies (Down syndrome, Edwards syndrome). - NTDs are **structural malformations**, not chromosomal abnormalities, though some chromosomal disorders may have associated structural defects. - Does not directly diagnose NTDs. *Placentography* - **Placentography** is used to localize the **placenta** in cases of suspected **placenta previa** or for guiding invasive procedures. - Provides no information about **fetal anatomy** and is not used for detecting NTDs.
Question 5: Number of chromosomes in Klinefelter syndrome:
- A. 44
- B. 46
- C. 47 (Correct Answer)
- D. 45
Explanation: ***47*** - **Klinefelter syndrome** is a genetic condition in males characterized by the presence of an extra X chromosome, resulting in a **47, XXY karyotype** [1]. - This additional chromosome increases the total count from the typical 46 to **47** [1]. *44* - A count of 44 chromosomes would indicate either a severe **aneuploidy** or a **haploid** state, which is not compatible with human life in a full somatic cell. - Normal human somatic cells contain 46 chromosomes (2n). *46* - A count of 46 chromosomes represents the **normal diploid number** for human somatic cells (46, XX for females and 46, XY for males). - This count would signify a genetically typical individual, not someone with Klinefelter syndrome. *45* - A count of 45 chromosomes typically indicates a **monosomy**, such as **Turner syndrome** (45, X) in females, where one sex chromosome is missing. - This is a different chromosomal abnormality from Klinefelter syndrome, which involves an extra chromosome. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 92-93.
Question 6: Nuchal translucency is used in
- A. Head scan
- B. MRI neck
- C. ANC USG (Correct Answer)
- D. Anthropometry
Explanation: ***ANC USG*** - **Nuchal translucency** measurement is a key component of the **first-trimester antenatal ultrasound** (ANC USG). - It helps in screening for chromosomal abnormalities like **Down syndrome (Trisomy 21)** and certain cardiac defects. *Head scan* - A head scan (e.g., CT or MRI of the head) is used to evaluate the **brain** and **skull** for conditions like tumors, strokes, or trauma. - It is not routinely used for fetal screening or nuchal translucency assessment. *MRI neck* - **MRI of the neck** is employed to visualize soft tissues, blood vessels, and bones in the neck region. - Its primary use is in diagnosing conditions like cervical disc herniations, spinal cord compression, or neck masses, not for fetal screening. *Anthropometry* - **Anthropometry** involves the measurement of the human body, such as height, weight, and limb circumference. - While general measurements are taken during pregnancy, anthropometry specifically does not refer to the assessment of nuchal translucency.
Question 7: Most appropriate time for chorionic villus sampling in pregnancy is:
- A. 16-18 weeks
- B. 16-20 weeks
- C. 8-10 weeks
- D. 10-13 weeks (Correct Answer)
Explanation: ***10-13 weeks*** - Chorionic villus sampling (CVS) is optimally performed between **10 and 13 completed weeks of gestation**. - This timing allows for **earlier diagnosis** of chromosomal abnormalities compared to amniocentesis, enabling more time for decision-making. *16-18 weeks* - This timeframe is typically used for **amniocentesis**, which is performed later in pregnancy. - Performing CVS at this stage is **outside the optimal window** and carries a higher risk profile for CVS specifically. *16-20 weeks* - This period is also generally recommended for **amniocentesis**, not chorionic villus sampling. - Delaying CVS until this point **reduces the advantage** of early diagnosis and may increase procedural risks. *8-10 weeks* - Performing CVS **before 10 weeks of gestation** is associated with a **higher risk of limb reduction defects** in the fetus. - Due to this significant risk, CVS is generally **contraindicated** before 10 weeks.
Question 8: Most commonly used sonological indicator for aneuploidy in first trimester is:
- A. Nuchal translucency (Correct Answer)
- B. Gestational sac volume
- C. Serum PAPP-A level
- D. Crown-rump length
Explanation: ***Nuchal translucency*** - **Nuchal translucency (NT)** is the most common and established sonographic marker for **Down syndrome** and other aneuploidies in the first trimester. - Increased NT thickness is directly correlated with a higher risk of **chromosomal abnormalities** and congenital heart defects. *Gestational sac volume* - **Gestational sac volume** is primarily used to assess the viability and proper development of the early pregnancy, not as a direct marker for aneuploidy. - Abnormalities in gestational sac size can indicate a **non-viable pregnancy** or **blighted ovum**, but not specific chromosomal defects. *Serum PAPP-A level* - **Serum PAPP-A (pregnancy-associated plasma protein-A)** is a biochemical marker used in conjunction with NT in first-trimester screening. - While it is part of aneuploidy risk assessment, **PAPP-A is a blood test**, not a sonological indicator itself. *Crown-rump length* - **Crown-rump length (CRL)** is used for **accurate dating of pregnancy** in the first trimester. - While an abnormal CRL can sometimes be associated with adverse pregnancy outcomes, it is not a direct or primary sonological indicator for aneuploidy screening.
Question 9: 18 weeks pregnant female presents with no high risk of NTD and low risk of trisomy 21 on quad test. What is the most appropriate next step in management?
- A. Repeat non-invasive screening test.
- B. Perform invasive diagnostic testing.
- C. Perform amniotic fluid analysis.
- D. Perform a detailed fetal ultrasound. (Correct Answer)
Explanation: ***Perform a detailed fetal ultrasound.*** - A **detailed fetal ultrasound** (often referred to as an **anatomy scan**) at around 18-22 weeks is a standard component of prenatal care for all pregnant women, regardless of screening test results. - This ultrasound evaluates fetal anatomy for structural anomalies, assesses fetal growth, and confirms gestational age, providing crucial information even with low-risk screening. *Repeat non-invasive screening test.* - Repeating a non-invasive screening test (like another quad screen or NIPT) is generally **not indicated** when initial results show a low risk and there are no other clinical concerns. - Such tests are primarily for screening purposes, and a second low-risk result would offer little additional actionable information, as their positive predictive value is low. *Perform invasive diagnostic testing.* - **Invasive diagnostic testing**, such as **amniocentesis** or **chorionic villus sampling (CVS)**, carries a risk of miscarriage and is reserved for situations with a high risk of chromosomal abnormalities or genetic conditions. - Given the low-risk quad screen results for trisomy 21 and no high risk for NTDs, invasive testing is **not warranted** at this stage. *Perform amniotic fluid analysis.* - **Amniotic fluid analysis** is part of an amniocentesis, an **invasive diagnostic procedure** designed to detect chromosomal abnormalities or genetic disorders. - This procedure is typically reserved for cases where screening tests indicate a high risk or there is a clinical suspicion of a genetic condition; it's **not a routine step** after a low-risk quad screen.
Question 10: Which of the following statements about chorionic villus sampling is false?
- A. Can cause limb deformities
- B. Is used for prenatal genetic diagnosis
- C. Villi are collected from chorion frondosum
- D. Is performed only in second trimester of pregnancy (Correct Answer)
Explanation: ***Is performed only in second trimester of pregnancy*** - This statement is false because **chorionic villus sampling (CVS)** is typically performed earlier in pregnancy, specifically during the **first trimester**, usually between 10 and 13 weeks of gestation. - Performing CVS only in the second trimester would negate one of its main advantages: providing earlier genetic diagnostic information than **amniocentesis**. *Is used for prenatal genetic diagnosis* - **CVS** is a primary method for **prenatal genetic diagnosis**, allowing for the detection of chromosomal abnormalities and genetic disorders. - It involves analyzing fetal cells obtained from the **chorionic villi**. *Villi are collected from chorion frondosum* - The sample for **CVS** is indeed collected from the **chorion frondosum**, which is the fetal part of the placenta containing numerous chorionic villi. - These villi are genetically identical to the fetus, making them suitable for **genetic analysis**. *Can cause limb deformities* - There is a recognized, albeit small, risk of **limb reduction defects** associated with CVS, particularly if performed very early in gestation (before 9-10 weeks). - This risk is part of the counseling provided to prospective parents considering the procedure.
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