Step 1: Identify the type of sentence.
The given sentence is a compound sentence, consisting of two independent clauses. We need to combine it into a simple sentence.
Step 2: Combine the sentences.
The most suitable way to combine the two parts of the sentence is by using an infinitive ("too weak to") to form a simple sentence.
Step 3: Final Conversion.
Thus, the correct simple sentence is: "The patient is too weak to walk upstairs."
Final Answer: \[ \boxed{\text{The patient is too weak to walk upstairs.}} \]