Five jumbled-up sentences (labelled 1, 2, 3, 4 and 5) related to a topic are given below. Four of them can be put together to form a coherent paragraph. Identify the odd sentence and key in the number of that sentence as your answer.

1. To create a synapse, the neuron has specialized structures, often seen as tiny swellings, at its terminal end of the axon where it stores the chemicals that are emitted to transmit a signal to the next neuron.

2. This fetal warm-up act—the soldering of neural connections before the eyes actually function—is crucial to the performance of the visual system.

3. The reasons for this paring back of synapses is a mystery, but synaptic pruning is thought to sharpen and reinforce the “correct” synapses, while removing the weak and unnecessary ones.

4. Neural connections between the eyes and the brain are formed long before birth, establishing the wiring and the circuitry that allow a child to begin visualizing the world the minute she emerges from the womb.

5. During this rehearsal period, synapses—points of chemical connection—between nerve cells are generated in great excess, only to be pruned back during later development.

A good starting point here would be to link sentences 5 and 3 based on the idea of “pruning.” Sentence 5 talks about the synaptic development that occurs during the early period described in sentences 4 and 2. It describes how synapses are initially overproduced, a concept that sets up the next part of the process: synaptic pruning. Sentence 3 continues this discussion by offering an explanation for why synapses are pruned: to reinforce the necessary connections and eliminate the weaker ones. This helps to fine-tune the brain’s wiring, ensuring that the important pathways are retained while the less useful ones are discarded. Together, 5-3 ties in with the broader purpose of neural wiring presented in Sentence 2: it emphasises that the creation of neural connections in the fetus is a preparatory stage and that these connections are essential for the future performance of the visual system. Sentence 4 sets the context of the paragraph: it mentions the formation of neural connections before birth, specifically focusing on the connections between the eyes and the brain.

While Sentence 1 provides a technical description of synapse creation and function, it doesn’t fit the broader context set by sentences 4, 2, 5, and 3. These sentences focus on brain development (especially related to vision) and the process of synapse formation and pruning in a developmental context. Sentence 1 focuses on the structural aspect of synapse formation (the “specialized structures” at the terminal end of an axon), which is more about how a synapse functions at a microscopic level rather than the larger developmental or functional role synapses play in early brain development, as discussed in the other sentences.

Therefore, Sentence 1 is the odd one out.