句子文法問題

Very rarely do food allergies cause patients to suffer a symptom more severe than that of migraine headaches.
句子的意思是:
在罕見的情況下﹐由食物敏感引起的症狀比偏頭痛的嚴重。

句子環繞兩種疾病的症狀 -- symptom. 句子的前段先提及食物敏感的症狀:
Very rarely do food allergies cause patients to suffer a symptom

句子的下段講述偏頭痛的症狀在一般情況下比食物敏感的嚴重﹐亦即是”比較“兩重症狀的輕重:
more severe than that of migraine headaches

在“比較”的句子中﹐由於需要比較的事物(symptom)於句子前段已出現過一次﹐為避免重複﹐symptom於句子後段可以省略﹐並用“that of”代替:
Very rarely do food allergies cause patients to suffer a symptom more severe than the symptom of that of migraine headaches.

“that of”在比較句子中有隱藏的功能﹐可免去重複詞彙的問題﹐又可清楚帶出原意。中文文法亦有相類用法﹐請看下例:
例一﹕
原句﹕小明的房子比小強的房子大。
改成﹕小明的房子比小強的房子大。 <-- 省去重複且意思相同的詞

英譯:
原句﹕Ming's house is bigger than Keung's house.
改成﹕Ming's house is bigger than house that of Keung's.

“that of”代替了句子前段出現過的“house”。“that of Keung's”意思為“小強的那個”。由於句子正比較小明及小強的房子﹐所以“小強的那個”在同一句子中當然是指“house”。

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suffer a symptom more severe than migraine headaches. （咁樣唔岩？）
如果將句子改為:
Very rarely do food allergies cause patients to suffer a symptom more than migraine headaches.

句子只能指出食物敏感的症狀比偏頭痛嚴重﹐但究竟比偏頭痛的甚麼嚴重呢? 句子並無清析說明。即使中譯後的意思亦不完整:
在罕見的情況下﹐由食物敏感引起的症狀比偏頭痛的嚴重。

省去了這個的字﹐令人難以明白正在比較的是“由食物敏感引起的症狀”及偏頭痛的“甚麼”? 究竟是指偏頭痛的發病率? 還是其他?

故此﹐句子必須寫成:
Very rarely do food allergies cause patients to suffer a symptom more severe than the symptom of migraine headaches.

省略後:
Very rarely do food allergies cause patients to suffer a symptom more severe than that of migraine headaches.

Wa! 詳細, good

that = a symptom
that of migraine headaches = a symptom of migraine headaches

the meaning :
很少情況下 :
a symptom of food allergies is more severe than a symptom of migraine headaches .

2007-04-03 04:46:15 補充：
...suffer a symptom more severe than migraine headaches. （咁樣唔岩？） 唔岩 .因為 severe ( 嚴重的 ) 是指 a symptom ( 症狀 )