Developmental Anatomy Practice Questions

Q: Which paranasal sinuses are present at birth?

Ethmoid and maxillary. The development of paranasal sinuses is a high-yield topic in embryology. At birth, only the **maxillary** and **ethmoid** sinuses are present and pneumatized enough to be clinically identifiable, though they are rudimentary in size. 1. **Maxillary Sinus:** This is the first sinus to develop (around the 3rd month of fetal life). At birth, it is a small sac measuring approximately 7 x 4 x 4 mm. 2. **Ethmoid Sinus:** These air cells are present at birth and continue to enlarge during early childhood. **Analysis of Incorrect Options:** * **Frontal Sinus:** This sinus is **absent at birth**. It starts developing from the anterior ethmoidal cells around the age of 2 and is usually not radiologically visible until age 5–7. * **Sphenoid Sinus:** While a tiny evagination may exist at birth, it is effectively **absent/non-pneumatized**. It begins to invade the sphenoid bone around age 3 and reaches full development in adolescence. **High-Yield Clinical Pearls for NEET-PG:** * **Sequence of Development:** Maxillary → Ethmoid → Sphenoid → Frontal (Mnemonic: **M**y **E**xtra **S**pecial **F**riend). * **Radiology:** The frontal sinus is the last to appear on an X-ray. * **Clinical Correlation:** Because the frontal and sphenoid sinuses are absent at birth, sinusitis in infants typically only involves the ethmoid or maxillary sinuses. * **Growth:** The maxillary sinus shows a rapid growth spurt during the eruption of permanent teeth.

Q: Which of the following is an example of a bone that is independent phylogenetically but is now fused with another bone?

Coracoid process of scapula. ### Explanation This question tests the concept of **Atavistic Epiphyses**. In developmental anatomy, epiphyses are classified into four types based on their function and evolutionary history: Pressure, Traction, Atavistic, and Aberrant. **Why Option C is Correct:** The **Coracoid process of the scapula** is the classic example of an **Atavistic epiphysis**. Phylogenetically, it was an independent bone (the coracoid bone) in lower vertebrates (like reptiles and birds), where it connected the scapula to the sternum. In mammals, it has lost its independent status and fused with the scapula, appearing as a separate center of ossification during development. **Analysis of Incorrect Options:** * **Options A & B (Trochanter of femur & Tubercle of humerus):** These are examples of **Traction epiphyses**. They develop at the site of tendon insertions (e.g., gluteal muscles on the trochanter, rotator cuff on the tubercles) and are produced by the "tug" or pull of muscles. They are not independent bones phylogenetically. * **Option D (Anterior tubercle of talus):** This is generally part of the primary ossification of the bone. However, the *posterior* tubercle of the talus (specifically the lateral tubercle) can sometimes fail to fuse, forming an accessory bone called the **Os Trigonum**, but it is not classified as an atavistic epiphysis in the same context as the coracoid. **High-Yield NEET-PG Pearls:** 1. **Pressure Epiphysis:** Located at the ends of long bones; transmits body weight (e.g., Head of femur, Lower end of radius). 2. **Traction Epiphysis:** Formed due to muscle pull; does not take part in joint transmission (e.g., Trochanters, Tubercles, Epicondyles). 3. **Atavistic Epiphysis:** A bone that was independent in ancestors but is now fused (e.g., Coracoid process, Os trigonum). 4. **Aberrant Epiphysis:** Not always present (e.g., Epiphysis at the base of the 1st metacarpal or head of other metacarpals).

Q: The spinal cord in infants ends at which vertebral level?

L3. ### Explanation The spinal cord terminates at different levels throughout development due to the **discrepancy in growth rates** between the vertebral column and the spinal cord (the vertebral column grows faster and longer than the nervous tissue). **1. Why L3 is Correct:** In **infants/neonates**, the spinal cord (conus medullaris) typically ends at the level of the **L3 vertebra**. Because the vertebral column has not yet completed its rapid longitudinal growth compared to the spinal cord, the cord occupies a relatively lower position in the spinal canal than it does in adults. **2. Analysis of Incorrect Options:** * **L1 (Option A):** This is the standard level of termination in **adults**. In approximately 65% of adults, the cord ends at the lower border of L1. * **L2 (Option B):** This is the lower limit of the normal range for adults (L1–L2). It is not the standard level for infants. * **L4 (Option D):** This is the level of the spinal cord termination during the **early fetal period** (around the 3rd month). By birth, it has "ascended" to L3. **3. NEET-PG High-Yield Pearls:** * **Embryology:** At 8 weeks of gestation, the spinal cord extends the entire length of the vertebral canal. * **Clinical Correlation (Lumbar Puncture):** To avoid needle injury to the spinal cord, a lumbar puncture is performed at the **L3–L4 or L4–L5** interspace in adults, but must be performed at a lower level (typically **L4–L5**) in infants to account for the lower termination of the cord. * **Subarachnoid Space:** While the cord ends at L1 (adult) or L3 (infant), the dural sac and subarachnoid space end at **S2** in both.

Q: Phocomelia is defined as which of the following conditions?

Absence of long bones. **Explanation:** **Phocomelia** is a rare congenital skeletal malformation characterized by the **absence or severe shortening of the proximal long bones** (humerus, femur, radius, ulna, tibia, or fibula). The term is derived from the Greek words *phoke* (seal) and *melos* (limb), as the hands or feet are attached directly to the trunk, resembling the flippers of a seal. 1. **Why Option A is Correct:** In phocomelia, the long bones fail to develop properly during the 4th to 8th week of gestation. This results in the hands and feet being abnormally positioned close to the shoulders or hips. 2. **Why Other Options are Incorrect:** * **Absence of the brain:** This is known as **Anencephaly**, a neural tube defect. * **Reduplication of bones:** This refers to conditions like **Polydactyly** (extra digits) or **Dimelia** (duplication of a limb part). * **Absence of the heart:** This is known as **Acardia**, a rare condition usually seen in monozygotic twin pregnancies (TRAP sequence). **High-Yield Clinical Pearls for NEET-PG:** * **Thalidomide Tragedy:** Phocomelia is classically associated with the use of **Thalidomide** by pregnant women in the 1950s-60s for morning sickness. It acts as a teratogen by inhibiting angiogenesis. * **Amelia:** Complete absence of a limb. * **Meromelia:** Partial absence of a limb (Phocomelia is a type of meromelia). * **Critical Period:** Limb development occurs between **days 24 to 36** of gestation; exposure to teratogens during this window is most critical.

Q: Non-disjunction of chromosomes occurs in which stage of cell division?

Anaphase. ### Explanation **Correct Option: C. Anaphase** **Why it is correct:** Non-disjunction is the failure of homologous chromosomes (in Meiosis I) or sister chromatids (in Meiosis II or Mitosis) to separate properly [1]. The physical separation of genetic material occurs specifically during **Anaphase**. * In **Anaphase I**, homologous pairs are pulled to opposite poles. * In **Anaphase II/Mitosis**, sister chromatids are pulled apart. If this separation fails, one daughter cell receives an extra chromosome (trisomy) while the other lacks one (monosomy) [1]. **Why other options are incorrect:** * **A. Prophase:** This stage involves chromatin condensation, nuclear envelope breakdown, and spindle formation. Chromosomes have not yet aligned or begun separation. * **B. Metaphase:** Chromosomes align at the equatorial plate. While the "Spindle Assembly Checkpoint" occurs here to ensure proper attachment, the actual movement (and thus the failure of movement) happens in the subsequent phase. * **D. Telophase:** This is the final stage where nuclear membranes reform around the already separated sets of chromosomes. Non-disjunction has already occurred by this point. **Clinical Pearls & High-Yield Facts:** 1. **Most Common Cause:** Maternal non-disjunction during **Meiosis I** (specifically Oogenesis) is the most common cause of trisomies like Down Syndrome (Trisomy 21). 2. **Risk Factor:** Advanced maternal age is the strongest risk factor for non-disjunction due to the long "dictyotene" arrest of oocytes in Prophase I. 3. **Mosaicism:** If non-disjunction occurs during **mitosis** in early embryonic development (post-fertilization), it leads to mosaicism (two or more cell lines with different genotypes in one individual) [1]. 4. **Aneuploidy:** Non-disjunction is the primary mechanism behind numerical chromosomal aberrations (e.g., Klinefelter syndrome 47,XXY; Turner syndrome 45,X) [1].

Question 1

Which paranasal sinuses are present at birth?

Accepted Answer

Ethmoid and maxillary

Answer

Frontal and maxillary

Answer

Frontal and ethmoid

Answer

Sphenoid and ethmoid

Question 2

Which of the following is an example of a bone that is independent phylogenetically but is now fused with another bone?

Accepted Answer

Coracoid process of scapula

Answer

Trochanter of femur

Answer

Tubercle of humerus

Answer

Anterior tubercle of talus

Question 3

The spinal cord in infants ends at which vertebral level?

Accepted Answer

L3

Answer

L1

Answer

L2

Answer

L4

Question 4

Phocomelia is defined as which of the following conditions?

Accepted Answer

Absence of long bones

Answer

Absence of the brain

Answer

Reduplication of bones

Answer

Absence of the heart

Question 5

Non-disjunction of chromosomes occurs in which stage of cell division?

Accepted Answer

Anaphase

Answer

Prophase

Answer

Metaphase

Answer

Telophase

Developmental Anatomy — MCQs

Developmental Anatomy — MCQs

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