Curriculum

17 Sections
1548 Lessons
100 Days

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U01.01 Biochemistry
This biochemistry review for USMLE Step 1 covers essential topics including metabolism, enzymology, and molecular biology.
126
U01.02 Immunology
Prepare for USMLE Step 1 Immunology with a comprehensive review covering foundational concepts, immune system mechanisms, and key disorders to solidify your understanding and boost exam readiness.
42
U01.03 Microbiology
Review essential microbiology topics for the USMLE Step 1, covering key pathogens, mechanisms of disease, and antimicrobial principles critical for mastering infectious disease concepts.
172
U01.04 General Pathology
Explore key concepts in general pathology essential for the USMLE Step 1. This review covers fundamental principles, disease mechanisms, and clinical correlations to help you excel in your medical exam.
35
U01.05 Pharmacology
Master key pharmacology concepts for the USMLE Step 1 with a focus on drug mechanisms, therapeutic uses, adverse effects, and interactions essential for high-yield exam performance.
43
U01.06 Public Health Sciences
Review essential Public Health Sciences topics for the USMLE Step 1, covering epidemiology, biostatistics, health policy, and ethics to strengthen your understanding of foundational public health principles.
53
U01.07 Cardiovascular system
Comprehensive review of essential cardiovascular system topics to help you excel in the USMLE Step 1 exam, covering key concepts from anatomy to pathology.
73
U01.08 Endocrine system
Master essential concepts of the endocrine system for USMLE Step 1, covering hormone regulation, feedback mechanisms, and key disorders to boost your exam readiness.
51
U01.09 Gastrointestinal system
Review key topics in the gastrointestinal system for the USMLE Step 1, covering essential concepts in anatomy, physiology, pathology, and pharmacology to reinforce your understanding and test readiness.
131
U01.10 Hematology and Oncology
This review covers essential Hematology and Oncology topics for the USMLE Step 1, including hematopoiesis, coagulation, anemias, leukemias, lymphomas, and neoplastic syndromes.
148
U01.11 Musculoskeletal, Skin, and Connective Tissue
Review essential topics for the USMLE Step 1 Musculoskeletal, Skin, and Connective Tissue systems, covering anatomy, physiology, pathology, and pharmacology fundamentals critical for exam success.
178
U01.12 Neurology and Special Senses
Master the foundational concepts of Neurology and Special Senses with our USMLE Step 1 review, covering essential topics like neuroanatomy, neurophysiology, and sensory pathways.
143
U01.13 Psychiatry
Psychiatry for the USMLE Step 1 focuses on understanding the pathophysiology, diagnosis, and treatment of psychiatric disorders, with an emphasis on key concepts like neurotransmitter imbalances, mental health disorders, and pharmacology.
83
U01.14 Renal System
The Renal System review for USMLE Step 1 covers key concepts including renal physiology, nephron function, acid-base balance, electrolyte regulation, and renal pathology, crucial for understanding both normal and abnormal kidney functions.
81
U01.15 Reproductive System
The USMLE Step 1 Reproductive System review covers key concepts in human reproduction, including anatomy, physiology, endocrinology, and common pathologies affecting both male and female reproductive health.
124
U01.16 Respiratory System
The Respiratory System review for the USMLE Step 1 covers key concepts in anatomy, physiology, pathophysiology, and pharmacology essential for understanding respiratory diseases and treatments.
61
U01.17 Rapid Review
he USMLE Step 1 Rapid Review covers essential topics across multiple disciplines, providing a concise, high-yield overview to reinforce key concepts and optimize your exam preparation.
5

U01.01.056 Autosomal trisomies

Autosomal trisomies result from the presence of an extra chromosome and are associated with characteristic phenotypic features and clinical findings. They include Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Patau syndrome (trisomy 13). These disorders are among the most commonly tested topics in the USMLE Step 1.

Trisomy 21 (Down Syndrome)

Key Features

Clinical Findings:
- Intellectual disability
- Flat facies
- Prominent epicanthal folds
- Single palmar crease
- Incurved 5th finger
- Gap between the 1st and 2nd toes
- Duodenal atresia, Hirschsprung disease
- Congenital heart defects (e.g., atrioventricular septal defect)
- Brushfield spots
- Early-onset Alzheimer disease (amyloid precursor protein on chromosome 21)
- Increased risk of acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML)

Epidemiology

Incidence: 1:700 live births
Etiology:
- 95%: Meiotic nondisjunction (increased with advanced maternal age: risk rises from 1:1500 in women <20 to 1:25 in women >45 years)
- 4%: Robertsonian translocation (e.g., between chromosomes 14 and 21)
- 1%: Post-fertilization mitotic error

Diagnosis

First-trimester ultrasound:
- Increased nuchal translucency
- Hypoplastic nasal bone
Biochemical Markers:
- 1st Trimester: Elevated β-hCG, decreased PAPP-A
- 2nd Trimester:
  - High: β-hCG, inhibin A
  - Low: Estriol, AFP

Mnemonic: The 5 A’s of Down Syndrome

Advanced maternal age
Atresia (duodenal)
Atrioventricular septal defect
Alzheimer disease (early onset)
AML/ALL

Trisomy 18 (Edwards Syndrome)

Key Features

Clinical Findings:
- PRINCE Edward:
  - Prominent occiput
  - Rocker-bottom feet
  - Intellectual disability
  - Nondisjunction
  - Clenched fists with overlapping fingers
  - Low-set Ears
- Micrognathia (small jaw)
- Congenital heart disease
- Omphalocele, myelomeningocele

Epidemiology

Incidence: 1:8000 live births
Prognosis: Death typically by age 1

Diagnosis

1st Trimester: Decreased β-hCG and PAPP-A
2nd Trimester:
- Low: β-hCG, estriol, AFP

Trisomy 13 (Patau Syndrome)

Key Features

Clinical Findings:
- Severe intellectual disability
- Rocker-bottom feet
- Microphthalmia, microcephaly
- Cleft lip/palate
- Holoprosencephaly
- Polydactyly
- Cutis aplasia (absence of skin)
- Congenital heart disease
- Polycystic kidney disease
- Omphalocele

Epidemiology

Incidence: 1:15,000 live births
Prognosis: Death typically by age 1

Diagnosis

1st Trimester: Decreased β-hCG and PAPP-A
2nd Trimester: No significant changes in β-hCG, estriol, inhibin A, or AFP

Pathogenesis

Defect in the fusion of prechordal mesoderm → midline defects

Comparison Table: Key Differences in Autosomal Trisomies

Feature	Trisomy 21 (Down Syndrome)	Trisomy 18 (Edwards Syndrome)	Trisomy 13 (Patau Syndrome)
Incidence	1:700	1:8000	1:15,000
Facial Features	Flat facies	Micrognathia	Microphthalmia, cleft lip/palate
Hand Features	Single palmar crease	Clenched fists, overlapping fingers	Polydactyly
Heart Defects	Atrioventricular septal defect	Congenital heart disease	Congenital heart disease
Prognosis	Viable	Death by age 1	Death by age 1

Points to Remember

Trisomy 21 is the most common viable autosomal trisomy, associated with advanced maternal age.
Trisomy 18 (Edwards) and Trisomy 13 (Patau) are associated with severe malformations and typically result in death within the first year of life.
Understanding biochemical markers and ultrasound findings for first- and second-trimester screening is crucial for USMLE Step 1 preparation.
Nondisjunction during meiosis is the most common cause of trisomies.

Bibliography

First Aid for the USMLE Step 1
Robbins and Cotran Pathologic Basis of Disease
UpToDate: “Overview of Chromosomal Anomalies”

USMLE Step 1 Review Series

Curriculum

U01.01.056 Autosomal trisomies

Trisomy 21 (Down Syndrome)

Key Features

Epidemiology

Diagnosis

Mnemonic: The 5 A’s of Down Syndrome

Trisomy 18 (Edwards Syndrome)

Key Features

Epidemiology

Diagnosis

Trisomy 13 (Patau Syndrome)

Key Features

Epidemiology

Diagnosis

Pathogenesis

Comparison Table: Key Differences in Autosomal Trisomies

Points to Remember

Bibliography

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