The sentence **"The doctor phoned the nurse because she was late."** is **ambiguous**; it can refer to either the doctor or the nurse as being late.

Let’s break it down:

- "**She**" is a pronoun that could refer to either "**the doctor**" or "**the nurse**", both of whom could be female.
- **Syntactic preference**: According to linguistic research (such as the work of Hobbs, 1979 and others), pronouns tend to refer to the most recently mentioned noun phrase of appropriate gender (the "recency effect"). In this sentence, "**the nurse**" is the closest feminine antecedent to "she", so many people would interpret "she" as referring to the nurse.
- **Real-world knowledge**: It's also plausible for people to infer who was late from context. For example, some might reason that nurses report to doctors more often than vice versa, so the nurse was late and the doctor called to ask why.

### Conclusion:

**Based purely on sentence structure, "she" most likely refers to "the nurse".**  
**But the sentence is ambiguous without more context.**

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**References:**
- Hobbs, J. R. (1979). “Resolving Pronoun References”. Lingua, 49(4): 311–338.
- Immediate antecedent rule in pronoun resolution.

If you meant to test how pronoun reference works in English grammar—**this is a classic example of an ambiguous pronoun!**
