A genetic counselor sees a family for the first time for genetic assessment. The 24-year-old businessman and his 19-year-old sister are concerned about having a mutant allele and have decided to get tested. Their grandfather and great aunt both have Huntington’s disease which became apparent when they turned 52. Their father who is 47 years old appears healthy. The geneticist discusses both the benefits and risks of getting tested and orders some tests. Which of the following tests would best provide evidence for whether the siblings are carriers or not?

Restriction enzyme digestion products

A 35-year-old woman gravida 2, para 1, comes to the physician for her first prenatal visit. Pregnancy and delivery of her first child were uncomplicated. She is not sure about the date of her last menstrual period. Pelvic examination shows a uterus consistent in size with a 10-week gestation. An ultrasound examination confirms the gestational age and shows one fetus with no indication of multiple gestations. During counseling on pregnancy risks and possible screening and diagnostic tests, the patient states she would like to undergo screening for Down syndrome. She would prefer immediate and secure screening with a low risk to herself and the fetus. Which of the following is the most appropriate next step in management at this time?

Nuchal translucency, pregnancy-associated plasma protein-A, human chorionic gonadotropin

Maternal serum α-fetoprotein, human chorionic gonadotropin, unconjugated estriol, and inhibin A

A 27-year-old G1P0 female presents for her first prenatal visit. She is in a monogamous relationship with her husband, and has had two lifetime sexual partners. She has never had a blood transfusion and has never used injection drugs. Screening for which of the following infections is most appropriate to recommend this patient?

Syphilis, HIV, HBV, and chlamydia

No routine screening is recommended for this patient

A 46-year-old woman presents to her primary care physician for her annual examination. At her prior exam one year earlier, she had a Pap smear which was within normal limits. Which of the following health screenings is recommended for this patient?

Blood glucose and/or HbA1c screening

Blood pressure at least once every 3 years

A 34-year-old gravida 2 para 1 woman at 16 weeks gestation presents for prenatal care. Her prenatal course has been uncomplicated. She takes no medications besides her prenatal vitamin which she takes every day, and she has been compliant with routine prenatal care. She has a 7-year-old daughter who is healthy. The results of her recent quadruple screen are listed below: AFP: Low hCG: Low Estriol: Low Inhibin-A: Normal Which of the following is the most appropriate next step to confirm the diagnosis?

Ultrasound for nuchal translucency

A 46-year-old man presents with increasing fatigue and weakness for the past 3 months. He works as a lawyer and is handling a complicated criminal case which is very stressful, and he attributes his fatigue to his work. He lost 2.3 kg (5.0 lb) during this time despite no change in diet or activity level. His past history is significant for chronic constipation and infrequent episodes of bloody stools. Family history is significant for his father and paternal uncle who died of colon cancer and who were both known to possess a genetic mutation for the disease. He has never had a colonoscopy or had any genetic testing performed. Physical examination is significant for conjunctival pallor. A colonoscopy is performed and reveals few adenomatous polyps. Histopathologic examination shows high-grade dysplasia and genetic testing reveals the same mutation as his father and uncle. The patient is concerned about his 20-year-old son. Which of the following is the most appropriate advice regarding this patient's son?

A genetic test followed by colonoscopy for the son should be ordered.

The son doesn't need to be tested now.

An immediate colonoscopy should be ordered for the son.

Screening can be started by 50 years of age as the son’s risk is similar to the general population.

The son should undergo a prophylactic colonic resection.

Carrier screening for genetic disorders for USMLE Step 3: High-Yield OB/GYN Lessons

Carrier Screening - Genetic Peek-a-Boo

Who: Offered to all women of reproductive age, ideally before conception.
What: Identifies asymptomatic carriers for autosomal recessive & X-linked disorders.
Core Panel: All patients offered screening for:
- Cystic Fibrosis
- Spinal Muscular Atrophy
- Hemoglobinopathies (e.g., Sickle Cell, Thalassemia)
Expanded Screening: Based on ethnicity or family history (e.g., Tay-Sachs, Fragile X).

⭐ Expanded carrier screening (ECS) panels, which test for hundreds of disorders simultaneously, are becoming more common in clinical practice.

Autosomal Recessive Inheritance Pedigree

Common Disorders - The Usual Suspects

Universal Screening Recommended (ACOG): All patients should be offered screening for Cystic Fibrosis and Spinal Muscular Atrophy, regardless of race or ethnicity.
Targeted Screening: Based on ethnicity or family history.

Disorder	Inheritance	High-Risk Populations
Cystic Fibrosis	Autosomal Recessive (AR)	Caucasians (Northern European), Ashkenazi Jewish
Spinal Muscular Atrophy	Autosomal Recessive (AR)	Pan-ethnic, but higher in Caucasians
Hemoglobinopathies	Autosomal Recessive (AR)	African, Mediterranean, Middle Eastern, Southeast Asian, West Indian
- Sickle Cell Disease
- Thalassemias
Tay-Sachs Disease	Autosomal Recessive (AR)	Ashkenazi Jewish, French-Canadian, Cajun
Fragile X Syndrome	X-linked Dominant	Associated with family history of intellectual disability, premature ovarian insufficiency

⭐ High-Yield: Carrier screening for Cystic Fibrosis and Spinal Muscular Atrophy should be offered to ALL pregnant individuals or those considering pregnancy, as they are among the most common severe inherited disorders.

Screening & Counseling - The Game Plan

Offer to all: Screen every woman, ideally preconception or in early pregnancy, regardless of ethnicity.
Universal Panel: Core screening includes Cystic Fibrosis (CF), Spinal Muscular Atrophy (SMA), and Hemoglobinopathies (CBC & electrophoresis).
Process: If the woman is a carrier, test her partner. If both are carriers, provide genetic counseling and offer prenatal diagnostic options.

⭐ If a patient’s carrier status is unknown but her partner is a known carrier of an autosomal recessive condition, the fetal risk is the mother's background carrier risk × 1/4.

High‑Yield Points - ⚡ Biggest Takeaways

Carrier screening should be offered to all women of reproductive age, ideally before conception.

Screening for Cystic Fibrosis (CF) and Spinal Muscular Atrophy (SMA) is recommended for all patients.

Additional screening is based on ethnicity or family history (e.g., sickle cell disease, thalassemias, Tay-Sachs).

If the mother is a carrier, the father of the baby should be tested next.

If both parents are carriers, genetic counseling is offered to discuss risks and prenatal diagnostic testing options.

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