Learn Russian: У пациента болит кость.

In Russian, physical pain is expressed backwards compared to English. The body part (кость) is the actual subject doing the action (the hurting), so it is in the nominative case. The person experiencing the pain is expressed using the У + genitive case construction (у пациента), meaning literally "at the patient, the bone hurts."

Why is the verb "болит" singular when it is referring to the patient's pain?

The verb болит always agrees with the body part that is hurting, not the person. Since кость (bone) is a singular noun, the verb is singular. If the patient had multiple bones hurting, the noun would be plural (кости) and the verb would change to the plural form болят.

How do we know the gender of "кость" since it ends in a soft sign?

Кость is a feminine noun. While you often have to memorize the gender of words ending in a soft sign (-ь), body parts ending this way (like кость, кровь, грудь) are almost always feminine and belong to the 3rd declension. Because of this, its plural form is кости.

Could we change the word order to "Кость болит у пациента"?

Yes, because Russian word order is flexible, but it changes the focus of the sentence. Starting with У пациента is the most natural, standard way to state who is in pain. Putting Кость first would strongly emphasize the bone, as if clarifying that the bone hurts rather than a muscle or a joint.