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Multisystem Genetics — Step 1 practice questions
39 questions in this area, with an explanation for every answer choice.
Three samples below, free and without an account.
How this shows up on Step 1
Genetics questions in this section combine two things the exam tests separately elsewhere. Inheritance patterns, pedigree interpretation, and the molecular basis of named conditions. This section covers what does not sit in one organ: shock, sepsis, poisoning, nutritional deficiency, and the genetic syndromes that touch several systems at once.
Sample questions
Sample question 1
A patient has hyperextensible, fragile skin, joint hypermobility, and easy bruising, with a defect in collagen synthesis. Which of the following is the most likely diagnosis?
Explanations
A. Marfan syndrome
Marfan syndrome results from a FIBRILLIN (not primarily collagen) defect, causing a different phenotype (tall stature, lens dislocation, aortic dilation) without this classic skin hyperextensibility/fragility pattern.
B. Osteogenesis imperfecta exclusively
Osteogenesis imperfecta also involves collagen defects but primarily manifests as bone fragility/fractures, not primarily this skin hyperextensibility pattern.
C. Down syndrome
Down syndrome is a chromosomal trisomy, unrelated to this collagen defect presentation.
D. Klinefelter syndrome
Klinefelter syndrome is a sex chromosome abnormality, unrelated to this collagen defect presentation.
E. Ehlers-Danlos syndrome · correct
Ehlers-Danlos syndrome, resulting from various collagen synthesis/structure defects, classically causes hyperextensible fragile skin, joint hypermobility, and easy bruising/poor wound healing.
Sample question 2
A 24-year-old man with thin translucent skin, easy bruising, and characteristic facial features presents with sudden severe abdominal pain and hypotension; imaging shows a ruptured splenic artery. His father died of a bowel perforation at age 40. Which of the following collagen types is defective?
Explanations
A. Type II collagen
Type II collagen is in cartilage and vitreous (defective in some chondrodysplasias).
B. Type IV collagen
Type IV is basement membrane collagen — Alport syndrome and Goodpasture antigen.
C. Type I collagen
Type I collagen defects cause osteogenesis imperfecta (fractures, blue sclerae, hearing loss) and the classic hypermobile/classical EDS overlap.
D. Type V collagen
Type V defects cause classical EDS with skin hyperextensibility and joint hypermobility, without arterial rupture.
E. Type III collagen · correct
Vascular Ehlers-Danlos syndrome (COL3A1 mutation) impairs type III collagen — the reticular collagen of blood vessels, bowel, and uterus — producing arterial, intestinal, and uterine rupture with translucent skin and minimal joint hypermobility. It is the lethal subtype; avoid invasive vascular procedures where possible.
Sample question 3
Both parents of a child with cystic fibrosis are unaffected carriers. What is the probability that their next child will also have cystic fibrosis?
Explanations
A. 25% · correct
For an autosomal recessive condition, two unaffected carrier (heterozygous) parents have a 25% (1 in 4) chance of having an affected (homozygous recessive) child with each pregnancy, according to a standard Punnett square (Aa x Aa → 1 AA : 2 Aa : 1 aa).
B. 0%
This does not reflect the correct autosomal recessive inheritance probability.
C. 75%
This does not reflect the correct probability.
D. 100%
This is not the correct probability for two heterozygous carrier parents.
E. 50%
This would be the probability of a child being a CARRIER (heterozygous), not the probability of being affected.
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