What's wrong with a three-and-a-half-month-old baby who cries and fusses severely?

　　A 3.5-month-old baby's severe crying may be caused by physiological factors, or pathological factors such as calcium deficiency, gastrointestinal dysfunction, or bacterial upper respiratory tract infection, which may require medication.

　　I. Physiological Factors

　　Physiological factors include high or low body surface temperature, insecurity, the need to urinate or defecate, wet diapers, crying due to hunger, inappropriate environmental temperature, or improper clothing. Parents should pay more attention to the baby's physiological condition, and soothing the baby's emotions can help alleviate these issues.

　　II. Pathological Factors

　　1. Calcium Deficiency

　　Calcium deficiency in babies can cause sweating, crying, being easily startled, and restless sleep. Crying caused by this reason can be addressed by oral administration of vitamin D capsules or vitamin A and D drops under a doctor's guidance, which can promote the absorption and utilization of calcium and phosphorus.

　　2. Gastrointestinal Dysfunction

　　Abdominal distension and discomfort caused by gastrointestinal dysfunction in babies can also lead to crying. Massaging the abdomen or applying heat compresses can help relieve discomfort. Additionally, under a doctor's guidance, oral administration of medications such as live Clostridium butyricum powder or live Bacillus subtilis bifidobacterium granules can be used to regulate gastrointestinal function.

　　3. Bacterial Upper Respiratory Tract Infection

　　Bacterial upper respiratory tract infection mainly presents with symptoms such as fever, abdominal pain, nasal congestion, and difficulty breathing. It may also cause fever, leading to crying in a 3-month-old baby. If it is a bacterial upper respiratory tract infection, treatment can include oral administration of medications such as azithromycin granules or roxithromycin granules. If the fever exceeds 38.5°C, timely physical cooling methods such as wiping with warm water and reducing clothing should be implemented. If necessary, medications such as acetaminophen granules or ibuprofen granules can be taken orally under a doctor's guidance to reduce fever.

　　In addition to the above causes, various surgical conditions can also cause baby crying, such as intussusception and intestinal colic.