Correct Answer: D. Osteogenesis imperfecta
Osteogenesis imperfecta (OI) is a genetic disorder of type I collagen synthesis characterized by severe bone fragility and multiple fractures. The prenatal presentation of multiple long bone fractures at 13 weeks gestation is pathognomonic for OI, particularly the severe forms (Type II—perinatal lethal, Type III—progressively deforming). The defective collagen cross-linking results in mechanically weak bone matrix that fractures with minimal or no trauma, even in utero from normal fetal movements. OI is inherited as an autosomal dominant condition (mutations in COL1A1 or COL1A2 genes). Prenatal ultrasound findings include multiple fractures, severe limb shortening, bowing deformities, and poor ossification. Type II OI is typically lethal in utero or perinatally due to severe skeletal dysplasia and pulmonary hypoplasia. The key discriminator is that OI is the ONLY condition among the options that presents with intrauterine fractures—the hallmark of severe bone fragility present from fetal life. This finding on second-trimester ultrasound is a classic indication for genetic counseling and fetal assessment in Indian obstetric practice.
Why the other options are wrong
A. Achondroplasia — Achondroplasia is a skeletal dysplasia affecting endochondral ossification, causing short stature and characteristic features (rhizomelic shortening, frontal bossing, lumbar lordosis). However, bone quality and strength are normal—fractures do NOT occur in utero or in infancy. The bones are short but dense and strong. This is a growth disorder, not a bone fragility disorder, making it incompatible with intrauterine fractures. B. Marfan syndrome — Marfan syndrome is a fibrillin-1 defect causing connective tissue laxity (tall stature, arachnodactyly, lens dislocation, aortic root dilatation). While skeletal manifestations are present, bone fragility and intrauterine fractures are NOT features. Marfan patients have normal bone strength; their skeletal problems relate to abnormal growth and joint hypermobility, not fracture risk in utero. C. Cretinism — Cretinism (congenital hypothyroidism) causes growth retardation, developmental delay, and skeletal immaturity, but does NOT cause intrauterine fractures. Bone fragility is not a feature of thyroid hormone deficiency. While skeletal maturation is delayed, bone quality remains normal. This is an endocrine disorder affecting growth, not bone matrix integrity.
High-Yield Facts
- Osteogenesis imperfecta Type II (perinatal lethal form) presents with intrauterine fractures, severe limb bowing, and poor ossification—lethal in utero or within hours of birth.
- Type I collagen defect in OI results from mutations in COL1A1/COL1A2 genes, causing defective collagen cross-linking and severe bone fragility from fetal life.
- Prenatal ultrasound findings in severe OI include multiple fractures, severe limb shortening, bowing deformities, and echogenic (poorly ossified) long bones at 13–20 weeks.
- Autosomal dominant inheritance in OI; 25% of cases are new mutations, important for genetic counseling in Indian families with affected pregnancies.
- Associated features in surviving OI patients include blue sclerae, dentinogenesis imperfecta, hearing loss, and progressive deformity—but intrauterine fractures are the prenatal hallmark.
Mnemonics
OI = Fragile Bones IN UTERO Osteogenesis Imperfecta = Fragile bones from fetal life. Think: defective collagen → weak matrix → fractures even before birth. Only OI among skeletal dysplasias causes prenatal fractures. Type II OI: "Perinatal LETHAL" Type II = severe intrauterine fractures + severe bowing + poor ossification = lethal in utero or within hours. If you see "intrauterine fractures," think Type II OI immediately.
NBE Trap
NBE pairs achondroplasia (the most common skeletal dysplasia) with bone fragility to trap students who confuse skeletal dysplasia with bone fragility disorders. Achondroplasia has short bones but STRONG bones; OI has fragile bones. The intrauterine fracture finding is the discriminator that eliminates all skeletal dysplasias.
Clinical Pearl
In Indian obstetric practice, detection of multiple intrauterine fractures on second-trimester ultrasound is an urgent indication for fetal karyotyping and genetic counseling. Type II OI is typically lethal; parents must be counseled about poor prognosis and offered termination options. Surviving OI patients (Types I, III, IV) require multidisciplinary care including orthopedic management, hearing assessment, and dental rehabilitation in Indian tertiary centers.
_Reference: Robbins Ch. 26 (Genetic Disorders); Harrison Ch. 346 (Heritable Disorders of Connective Tissue); OP Ghai Ch. 8 (Genetic Disorders in Pediatrics)_