**Core Concept:**
Infants have a longer spinal cord compared to adults due to their growing bodies. The spinal cord ends at different vertebral levels based on the individual's age.

**Why the Correct Answer is Right:**
In infants, the spinal cord extends up to the T12 or L1 vertebrae. This is because the spinal cord grows proportionally with the body's growth. As they mature, the spinal cord will retract and eventually end at the level of the first sacral vertebrae (S1). This process is known as "spinal cord maturation."

**Why Each Wrong Option is Incorrect:**
A. Spinal cord ends at T12 or L1 in infants: This is incorrect because the spinal cord continues beyond T12 or L1 in young infants.
B. Spinal cord ends at the first thoracic vertebrae (T1): This is incorrect because the spinal cord extends beyond T1 in infants.
C. Spinal cord ends at the first lumbar vertebrae (L1): Similar to option B, this choice is incorrect as the spinal cord grows with the infant's body.
D. Spinal cord ends at the first sacral vertebrae (S1): This is the correct answer as it represents the endpoint of the spinal cord during spinal cord maturation in infants.

**Clinical Pearl / High-Yield Fact:**
The process of spinal cord maturation is a crucial aspect of pediatric neurosurgery and neurology, as understanding this phenomenon helps in diagnosing and managing spinal cord-related conditions in infants more accurately. For example, an abnormal prolongation of the spinal cord may indicate a congenital malformation or injury, and prompt further evaluation and intervention.

**Correct Answer:** D. Spinal cord ends at the first sacral vertebrae (S1)

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To answer this multiple-choice question, we must first understand the concept of spinal cord maturation in infants. The spinal cord extends beyond the first sacral vertebrae (S1) in infants due to their rapid growth during the early stages of life. This process is known as spinal cord maturation, where the spinal cord gradually shortens as the child grows and develops. Understanding this process is essential for diagnosing and managing spinal cord-related disorders in infants accurately.